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

Ensure that the the default settings for the room directory are that the it is hidden from public view by default. 

(hopefully)

... and deobfuscate the relevant bit of code.

Implement part [MSC2228](https://github.com/matrix-org/matrix-doc/pull/2228). The parts that differ are:

* the feature is hidden behind a configuration flag (`enable_ephemeral_messages`)
* self-destruction doesn't happen for state events
* only implement support for the `m.self_destruct_after` field (not the `m.self_destruct` one)
* doesn't send synthetic redactions to clients because for this specific case we consider the clients to be able to destroy an event themselves, instead we just censor it (by pruning its JSON) in the database

Signed-Off-By: Filip Štědronský <g@regnarg.cz>

Cf #6422

Guess I only tested this on python 2 :/

Fixes #6419.

Guess I only tested this on python 2 :/

Fixes #6419.

Fix phone home stats

There are lots of words in the comment as to why this is a good idea.

Fixes #6403.

Add some exception handling here so that events whose json cannot be parsed are
ignored rather than getting us stuck in a loop.

Fixes #6404.

* Add working build command for docker image

* Add changelog