a couple of cleanups

Continuation of #7379

Adds a section in the README telling people to go to #synapse:matrix.org instead of using github issues. I'm not entirely sure about placing it above the install section but then people are likely to first seek support when installing (if something goes boom), and it's probably better to have it as high as possible anyway so people actually see it.

This PR moves the "support is in #synapse:matrix.org" in the bug report template outside of the comment as some people seem to ignore what's in the comments, and phrase it a bit more like the support request template. It also adds a default issue template that says the same thing. It's also adding a notice about the security disclosure to both the default template and the bug report one.

It also adds a badge to the top of the README with an alt text saying about the same message if the badge doesn't load (e.g. if matrix.org is slow).

Fixes #6826 

Synapse v1.12.4

Features:

* Always send users their own device updates. (#7160)
* Add support for handling GET requests for account_data on a worker. (#7311)

Bugfixes:

* Fix a bug that prevented cross-signing with users on worker-mode synapses. (#7255)
* Do not treat display names as globs in push rules. (#7271)
* Fix a bug with cross-signing devices belonging to remote users who did not share a
  room with any user on the local homeserver. (#7289)

This reverts commit 6f431936, reversing
changes made to 0d775fcc.

Config option to inhibit 3PID errors on /requestToken

Adds a request_token_inhibit_errors configuration flag (disabled by
default) which, if enabled, change the behaviour of all /requestToken
endpoints so that they return a 200 and a fake sid if the 3PID was/was
not found associated with an account (depending on the endpoint),
instead of an error.

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

I updated the PR and forgot to update the changelog.

This was incorrectly merged to the release branch before it was ready.

This reverts commit 72fe2aff.

Add changelog

Save retrieved keys to the db

lint

Fix and de-brittle remote result dict processing

Use query_user_devices instead, assume only master, self_signing key types

Make changelog more useful

Remove very specific exception handling

Wrap get_verify_key_from_cross_signing_key in a try/except

Note that _get_e2e_cross_signing_verify_key can raise a SynapseError

lint

Add comment explaining why this is useful

Only fetch master and self_signing key types

Fix log statements, docstrings

Remove extraneous items from remote query try/except

lint

Factor key retrieval out into a separate function

Send device updates, modeled after SigningKeyEduUpdater._handle_signing_key_updates

Update method docstring

Fixes #7144.

Synapse 1.12.3 (2020-04-03)
===========================

- Remove the the pin to Pillow 7.0 which was introduced in Synapse 1.12.2, and
correctly fix the issue with building the Debian packages. ([\#7212](https://github.com/matrix-org/synapse/issues/7212))

Synapse 1.12.2 (2020-04-02)
===========================

This release fixes [an
issue](https://github.com/matrix-org/synapse/issues/7208) with building the
debian packages.

No other significant changes since 1.12.1.

This was incorrectly merged to master.

This reverts commit 319c41f5, reversing
changes made to 229eb814.

This was incorrectly merged to `master` instead of develop.

This reverts commit 90246344.

Synapse 1.12.1 (2020-04-02)
===========================

No significant changes since 1.12.1rc1.

Synapse 1.12.1rc1 (2020-03-31)
==============================

Bugfixes
--------

- Fix starting workers when federation sending not split out. ([\#7133](https://github.com/matrix-org/synapse/issues/7133)). Introduced in v1.12.0.
- Avoid importing `sqlite3` when using the postgres backend. Contributed by David Vo. ([\#7155](https://github.com/matrix-org/synapse/issues/7155)). Introduced in v1.12.0rc1.
- Fix a bug which could cause outbound federation traffic to stop working if a client uploaded an incorrect e2e device signature. ([\#7177](https://github.com/matrix-org/synapse/issues/7177)). Introduced in v1.11.0.

* tag 'v1.12.1':
  1.12.1
  Note where bugs were introduced
  1.12.1rc1
  Newsfile
  Rewrite changelog
  Add changelog
  Only import sqlite3 when type checking
  Fix another instance
  Only setdefault for signatures if device has key_json
  Fix starting workers when federation sending not split out.

Fixes: #7127
Signed-off-by: David Vo <david@vovo.id.au>