Allow Synapse to send registration emails + choose Synapse or an external...

Allow Synapse to send registration emails + choose Synapse or an external server to handle 3pid validation (#5987)

This is a combination of a few different PRs, finally all being merged into `develop`:

* #5875 
* #5876 
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](https://github.com/matrix-org/synapse/commit/891afb57cbdf9867f2848341b29c75d6f35eef5a#diff-e591d42d30690ffb79f63bb726200891) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835 
* #5969 
* #5940

Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.

UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.