Due to #5269 we may have extremities in our DB that we shouldn't have,
so lets add a cleanup task such to remove those.

When we receive a soft failed event we, correctly, *do not* update the
forward extremity table with the event. However, if we later receive an
event that references the soft failed event we then need to remove the
soft failed events prev events from the forward extremities table,
otherwise we just build up forward extremities.

Fixes #5269

This reverts commit c31e375a.

Only count aggregations from distinct senders

Block attempts to annotate the same event twice

Implements https://github.com/matrix-org/matrix-doc/pull/2002.

When considering the candidates to be forward-extremities, we must exclude soft
failures.

Hopefully fixes #5090.

* Pin eliot to <1.8 on python 3.5.2

Fixes https://github.com/matrix-org/synapse/issues/5199

* Add support for 'markers' to python_dependencies

* tell xargs not to strip quotes

Add startup background job for account validity

If we remove support for a particular room version, we should behave more
gracefully. This should make client requests fail with a 400 rather than a 500,
and will ignore individiual PDUs in a federation transaction, rather than the
whole transaction.

This reverts commit ce5bcefc.

This caused:

```
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/synapse/src/synapse/app/client_reader.py", line 32, in <module>
    from synapse.replication.slave.storage import SlavedProfileStore
ImportError: cannot import name 'SlavedProfileStore' from 'synapse.replication.slave.storage' (/home/synapse/src/synapse/replication/slave/storage/__init__.py)
error starting synapse.app.client_reader('/home/synapse/config/workers/client_reader.yaml') (exit code: 1); see above for logs
```

Implements MSC1884

Rename relation types to match MSC

Land basic reaction and edit support.

Correctly update aggregation counts after redaction

Add basic editing support

If account validity is enabled in the server's configuration, this job will run at startup as a background job and will stick an expiration date to any registered account missing one.

As a user isn't allowed to send a single emoji more than once.

As users aren't allowed to react with the same emoji more than once.

* fix mapping of return values for get_or_register_3pid_guest