Manual device verification e.g. for bots like Mjolnir

[andybalaam]

When we start excluding unsigned devices then all users in a room, including bots, will need to verify their devices.

Some bots, e.g. mjolnir don't currently provide a way to verify their devices, and the underlying matrix-bot-sdk does not provide a way of doing this. The preferred way would be to supply a Recovery Key during bot setup, but without support in matrix-bot-sdk this would be quite a lot of development work to do.

So, we want an easier way of manually asserting that a device is verified. We think the simplest way would be for the bot to print out its ed25519 key on startup, so we can supply it to another client.

Probably in Element Web, we should provide a dialog (probably in an "Advanced" section) that allows pasting in a client's ed25519 key to verify it. Note: @Half-Shot pointed out that some people would find this much easier if it were part of the ESS management console or even a custom tool, rather than Element Web. On the other hand, for community use, putting it in ESS would not help.

[andybalaam]

This is a proposed solution to the problem described in #2709