More updates to Promethese metrics exposition

Replace postgres GIN with GIST

This is because GIN can be slow to write too, especially when the table
gets large.

Don't error on non-ascii passwords

Set CORs headers on responses from the media repo

Remove sent_transactions table.

Update documentation about exported prometheus metrics

This should further reduce the number of individual inserts,
transactions and updates that are required for keeping sent_transactions
up to date.

Remove long-deprecated instructions about promethesus console; also fix for modern config file format

Bugfix for process-wide metric export on split processes

Pass the Metrics group into the process collector instead of having it find its own one; this avoids it needing to import from synapse.metrics

Fix user_threepids schema delta

The delta `37/user_threepids.sql` aimed to update all the email
addresses to be lower case, however duplicate emails may exist in the
table already.

This commit adds a step where the delta moves the duplicate emails to a
new `medium` `email_old`. Only the most recently used account keeps the
binding intact. We move rather than delete so that we retain some record
of which emails were associated with which account.

Allow clients to supply access_tokens as headers