This is based on the work done by @krombel in #2601.

* Add note to UPGRADE.rst about removing riot.im from list of trusted identity servers

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

This fallback didn't work, and was removed in #4069.

We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

This requires the psutil module, and is still opt-in based on the report_stats
config option.

I've added some Documentation, how to get the running Version of a
Synapse homeserver. This should help the HS-Owners to check whether the
Upgrade was successful.