Make sure that synapse doesn't explode when a redaction redacts itself, or
there is a larger cycle.

* Fix servlet metric names

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Remove redundant check

* Cover all return paths

* Opentracing survival guide

* Update decorator names in doc

* Doc cleanup

These are all alterations as a result of comments in #5703, it
includes mostly typos and clarifications. The most interesting
changes are:

- Split developer and user docs into two sections
- Add a high level description of OpenTracing

* newsfile

* Move contributer specific info to docstring.

* Sample config.

* Trailing whitespace.

* Update 5703.misc

* Apply suggestions from code review

Mostly just rewording parts of the docs for clarity.

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Clean up config settings and dead code.

This is mostly about cleaning up the config format, to bring it into line with our conventions. In particular:
 * There should be a blank line after `## Section ##' headings
 * There should be a blank line between each config setting
 * There should be a `#`-only line between a comment and the setting it describes
 * We don't really do the `#  #` style commenting-out of whole sections if we can help it
 * rename `tracer_enabled` to `enabled`

While we're here, do more config parsing upfront, which makes it easier to use
later on.

Also removes redundant code from LogContextScopeManager.

Also changes the changelog fragment to a `feature` - it's exciting!

Fixes #5594

Forbid viewing relations on an event once it has been redacted.

* Convert BaseFederationServlet._wrap to async

Empirically, this fixes some lost stacktraces. It should be safe because the
wrapped function is called from JsonResource._async_render, which is already
async.

* Convert the rest of synapse.federation.transport.server to async

We may as well do the whole file while we're here.

* changelog

* flake8

This is basically a contrived way of adding a `Recommends` on `libpq5`, to fix #5653.

The way this is supposed to happen in debhelper is to run
`dh_shlibdeps`, which in turn runs `dpkg-shlibdeps`, which spits things out
into `debian/<package>.substvars` whence they can later be included by
`control`.

Previously, we had disabled `dh_shlibdeps`, mostly because `dpkg-shlibdeps`
gets confused about PIL's interdependent objects, but that's not really the
right thing to do and there is another way to work around that.

Since we don't always use postgres, we don't necessarily want a hard Depends on
libpq5, so I've actually ended up adding an explicit invocation of
`dpkg-shlibdeps` for `psycopg2`.

I've also updated the build-depends list for the package, which was missing a
couple of entries.

We can now use `_get_events_from_cache_or_db` rather than going right back to
the database, which means that (a) we can benefit from caching, and (b) it
opens the way forward to more extensive checks on the original event.

We now always require the original event to exist before we will serve up a
redaction.

Ensures that redactions are correctly authenticated for recent room versions.

There are a few things going on here:

 * `_fetch_event_rows` is updated to return a dict rather than a list of rows.

 * Rather than returning multiple copies of an event which was redacted
   multiple times, it returns the redactions as a list within the dict.

 * It also returns the actual rejection reason, rather than merely the fact
   that it was rejected, so that we don't have to query the table again in
   `_get_event_from_row`.

 * The redaction handling is factored out of `_get_event_from_row`, and now
   checks if any of the redactions are valid.

A couple of changes here:

* get rid of a redundant `allow_rejected` condition - we should already have filtered out any rejected
  events before we get to that point in the code, and the redundancy is confusing. Instead, let's stick in
  an assertion just to make double-sure we aren't leaking rejected events by mistake.

* factor out a `_get_events_from_cache_or_db` method, which is going to be important for a 
  forthcoming fix to redactions.

Create basic admin command app

Co-Authored-By: Aaron Raimist <aaron@raim.ist>

Return `This account has been deactivated` instead of `Invalid password` when a user is deactivated.

Add basic function to get all data for a user out of synapse