After https://github.com/matrix-org/synapse/pull/8377, the deb packages no longer indirectly installed the `"test"` dependencies, causing debian packages to fail to build while carrying out the unit tests.

This PR installs `test` dependencies explicitly when building debian packages.

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

#7124 changed the behaviour of remote thumbnails so that the thumbnailing method was included in the filename of the thumbnail. To support existing files, it included a fallback so that we would check the old filename if the new filename didn't exist.

Unfortunately, it didn't apply this logic to storage providers, so any thumbnails stored on such a storage provider was broken.

For negative streams we have to negate the internal stream ID before
querying the DB.

The effect of this bug was to query far too many rows, slowing start up
time, but we would correctly filter the results afterwards so there was
no ill effect.

This converts a few more of our inline HTML templates to Jinja. This is somewhat part of #7280 and should make it a bit easier to customize these in the future.

Introduced in #8236

we're not doing a final release yet!

Add a pair of federation metrics to track the delays in sending PDUs to/from 
particular servers.

The idea is that in future tokens will encode a mapping of instance to position. However, we don't want to include the full instance name in the string representation, so instead we'll have a mapping between instance name and an immutable integer ID in the DB that we can use instead. We'll then do the lookup when we serialize/deserialize the token (we could alternatively pass around an `Instance` type that includes both the name and ID, but that turns out to be a lot more invasive).

Add an improved "forward extremities" metric

Hopefully, N(extremities) * N(state_events) is a more realistic approximation
to "how big a problem is this room?".

This was a bit unweildy for what I wanted: in particular, I wanted to assign
each measurement straight into a bucket, rather than storing an intermediate
Counter which didn't do any bucketing at all.

I've replaced it with something that is hopefully a bit easier to use.

(I'm not entirely sure what the difference between a HistogramMetricFamily and
a GaugeHistogramMetricFamily is, but given our counters can go down as well as
up the latter *sounds* more accurate?)

Our hacked-up `_exposition.py` was stripping out some samples it shouldn't
have been. Put them back in, to more closely match the upstream
`exposition.py`.

Drop compatibility hacks for prometheus-client pre 0.4.0. Debian stretch and
Fedora 31 both have newer versions, so hopefully this will be ok.

Report metrics on expensive rooms for state res