Implements MSC1884

Rename relation types to match MSC

Land basic reaction and edit support.

Correctly update aggregation counts after redaction

Add basic editing support

As users aren't allowed to react with the same emoji more than once.

* fix mapping of return values for get_or_register_3pid_guest

Make generating SQL bounds for pagination generic

* expose SlavedProfileStore to ClientReaderSlavedStore

* Stop telling people to install the optional dependencies.

They're optional.

Also update the postgres docs a bit for clarity(?)

Add an option to disable per-room profiles

Re-order registration stages to do msisdn & email auth last

Add relation aggregation APIs

Merge pull request #5187 from matrix-org/dbkr/only_check_threepid_not_in_use_if_actually_registering

Only check 3pids not in use when registering