Dendrite support

[Henrik]

It would be nice to have official support for Dendrite as well as Synapse, including migrating from the dendrite builtin media store to MMR.

Probably not a high priority before Dendrite reaches 1.0, but once it does, it would be very nice to have, especially considering Dendrite will have a migration path from Synapse to Dendrite in the future

[Travis Ralston]

This has been broken out to the following tasks:

• #496
• #497
• #498
• #499

To consider this issue "done", all but #498 need to be complete for Synapse parity.