Some notes:
- Doesn't re-parse reactions already in the db to add the url field - so
may need an initial sync for those.
- Since some clients don't really follow MSC3746, as in: they don't use
the url field, but instead only write and check the key if it is an
mxc-url, support those as well.
- Accordingly, initial sync is likely not required for those reactions
I've seen in the wild so far, as it's common to use the mxc url also
as key.
Change-Id: Ib1c50315425494986fa2e794d165658220a4f342
