Fix destructuring assumption bug with using load_jinja2_templates

Co-Authored-By: Erik Johnston <erik@matrix.org>

Don't assume there is a 'self' arg in @trace decorator

Allow Synapse to send registration emails + choose Synapse or an external server to handle 3pid validation (#5987)

This is a combination of a few different PRs, finally all being merged into `develop`:

* #5875 
* #5876 
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](https://github.com/matrix-org/synapse/commit/891afb57cbdf9867f2848341b29c75d6f35eef5a#diff-e591d42d30690ffb79f63bb726200891) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835 
* #5969 
* #5940

Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.

UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.

Link the send loop with the edus contexts

Co-Authored-By: Erik Johnston <erik@matrix.org>

Co-Authored-By: Erik Johnston <erik@matrix.org>

The contexts were being filtered too early so  the send loop wasn't
being linked to them unless the destination
was whitelisted.

Implements `POST /_matrix/client/r0/account/3pid/unbind` from [MSC2140](https://github.com/matrix-org/matrix-doc/blob/dbkr/tos_2/proposals/2140-terms-of-service-2.md#post-_matrixclientr0account3pidunbind).

Removes the `bind_email` and `bind_msisdn` parameters from the `/register` C/S API endpoint as per [MSC2140: Terms of Service for ISes and IMs](https://github.com/matrix-org/matrix-doc/pull/2140/files#diff-c03a26de5ac40fb532de19cb7fc2aaf7R107).

Very small code cleanup.

Previously the stats were not being correctly populated.

* Ensure the list media admin API is always available

This API is required for some external media repo implementations to operate (mostly for doing quarantine operations on a room).

* changelog

Adjust su-exec to only be used if needed.

If UID == getuid() and GID == getgid() then we do not need to su-exec, and chmod will not work.

* Ensure an auth instance is available to ListMediaInRoom

Fixes https://github.com/matrix-org/synapse/issues/5737

* Changelog

Remove all the "double return" statements which were a result of us removing all the instances of

```
defer.returnValue(...)
return
```

statements when we switched to python3 fully.

Trace device list changes.

These methods were part of the v1 C/S API. Remove them as they are no longer used by any code paths.

* fix thumbnail storage location

Signed-off-by: Lorenz Steinert <lorenz@steinerts.de>

* Add changelog file.

Signed-off-by: Lorenz Steinert <lorenz@steinerts.de>

* Update Changelog

Signed-off-by: Lorenz Steinert <lorenz@steinerts.de>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

This reverts commit 4765f0cf.

Python will return a tuple whether there are parentheses around the returned values or not.

I'm just sick of my editor complaining about this all over the place :)

Adds a flag to `/versions`' `unstable_features` section indicating that this Synapse understands what an `id_access_token` is, as per https://github.com/matrix-org/synapse/issues/5927#issuecomment-523566043

Fixes #5927