[Feature] Support i2p

[ghbjklhv]

Objective: Implement and Support i2p in Matrix and Riot.im
Description: Explore the possibility of using Matrix over i2p.
Would non-i2p users be able to still communicate? Would you need to craft a bridge?
Why?: Many reasons, mainly for privacy and security but also to support decentralized communications.

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[cyphar]

This is (at least somewhat) related to matrix-org/synapse#5152 -- the basic problem is that right now there isn't a way to proxy synapse federation requests (from what I can tell that might even be a Twisted issue). We also will probably need to have a way to force a room to only be join-able by anonymous servers (like we currently can block non-local users from joining a room). We'd also need to get a large number of servers to proxy (at the very least) .onion services over Tor in order to make sure that the .onion homeservers would be able to get events.

On the client-side there shouldn't be too many issues (you can always just open the client in Tor Browser or the I2P equivalent).

[Mikaela]

Also related to #200