Import / export e2e keys

[dessalines]

Now that e2e support is added, we need a way to import and export the keys from other clients.

[mujx]

The current implementation is incompatible with Riot due to missing spec, so it will only work between nheko instances.

Once the documentation is released we'll switch to it.

[ara4n]

is there any chance you might be able to help finalise the spec for this? otherwise i can see it dragging on (given spec bandwidth is otherwise focused entirely on s2s r0) and more pain like #445

[mujx]

@ara4n I doubt I can be of any help with it because I don't understand the algorithm in use. It would have been implemented otherwise.