Use m.bridge bridgebot info for DM avatar generation (MSC2346)

[api33tiramisu]

SchildiChat works well with Beeper server, and non-Beeper (bridged) messages are correctly placed in the People section instead of Rooms, but currently they use placeholder image instead of the user avatar.

[SpiritCroc]

Technical details as they come to mind from what I remember: In Beeper-DMs, there tends to be three members (you, the chat partner, and the bridge DM which handles decryption). Accordingly, normal matrix clients do not know which avatar image to use (by not being able to tell apart the bot user from the ghost user). Beeper addresses this by having a state event telling the client who is the bot user.

We could implement the same on SchildiChat, it basically boils down to more or less implementing partial support for MSC2346 as far as I see.