We can use the existing `_get_e2e_device_keys_and_signatures_txn` instead of
creating our own txn function

This is *not* ready for production yet. Caveats:

1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.

* Add shared_rooms api

* Add changelog

* Add .

* Wrap response in {"rooms": }

* linting

* Add unstable_features key

* Remove options from isort that aren't part of 5.x

`-y` and `-rc` are now default behaviour and no longer exist.

`dont-skip` is no longer required

https://timothycrosley.github.io/isort/CHANGELOG/#500-penny-july-4-2020



* Update imports to make isort happy

* Add changelog

* Update tox.ini file with correct invocation

* fix linting again for isort

* Vendor prefix unstable API

* Fix to match spec

* import Codes

* import Codes

* Use FORBIDDEN

* Update changelog.d/7785.feature

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

* Implement get_shared_rooms_for_users

* a comma

* trailing whitespace

* Handle the easy feedback

* Switch to using runInteraction

* Add tests

* Feedback

* Seperate unstable endpoint from v2

* Add upgrade node

* a line

* Fix style by adding a blank line at EOF.

* Update synapse/storage/databases/main/user_directory.py

Co-authored-by: Tulir Asokan <tulir@maunium.net>

* Update synapse/storage/databases/main/user_directory.py

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

* Update UPGRADE.rst

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

* Fix UPGRADE/CHANGELOG unstable paths

unstable unstable unstable

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Co-authored-by: Tulir Asokan <tulir@maunium.net>

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Tulir Asokan <tulir@maunium.net>

this makes it a bit clearer what's going on.

Fixes https://github.com/matrix-org/synapse/issues/8185

... to `_get_e2e_device_keys_and_signatures_txn`, to better reflect what it
does.

This is so that we can use it for the backfill events stream.

This is due to compatibility issues with old Python versions.

* Move `get_devices_with_keys_by_user` to `EndToEndKeyWorkerStore`

this seems a better fit for it.

This commit simply moves the existing code: no other changes at all.

* Rename `get_devices_with_keys_by_user`

to better reflect what it does.

* get_device_stream_token abstract method

To avoid referencing fields which are declared in the derived classes, make
`get_device_stream_token` abstract, and define that in the classes which define
`_device_list_id_gen`.

... and to show that it does something slightly different to
`_get_e2e_device_keys_txn`.

`include_all_devices` and `include_deleted_devices` were never used (and
`include_deleted_devices` was broken, since that would cause `None`s in the
result which were not handled in the loop below.

Add some typing too.

This fixes a bug where having multiple callers waiting on the same
stream and position will cause it to try and compare two deferreds,
which fails (due to the sorted list having an entry of `Tuple[int,
Deferred]`).

There's not much point in returning all the others, and some people have a
silly number of devices.

This is split out from https://github.com/matrix-org/synapse/pull/7438, which had gotten rather large.

`LoginRestServlet` has a couple helper methods, `login_submission_legacy_convert` and `login_id_thirdparty_from_phone`. They're primarily used for converting legacy user login submissions to "identifier" dicts ([see spec](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-login)). Identifying information such as usernames or 3PID information used to be top-level in the login body. They're now supposed to be put inside an [identifier](https://matrix.org/docs/spec/client_server/r0.6.1#identifier-types) parameter instead.

#7438's purpose is to allow using the new identifier parameter during User-Interactive Authentication, which is currently handled in AuthHandler. That's why I've moved these helper methods there. I also moved the refactoring of these method from #7438 as they're relevant.

This was forgotten in #8164.