E2E Crypto: key display

[Tomatower]

Having a e2e encryption is only useful if one can compare the keys, so it is necessary to add the option to display your device key as well as the key of your communication partners.

[vurpo]

I agree that this should be added to achieve full E2E functionality, but E2E does still have some use already now without the ability to see the key fingerprints. For example, it protects against all passive eavesdropping attacks even if you don't compare the key fingerprints.

[mujx]

This would be even more useful with a device list for each user (which probably should be part of a user profile page).

[unclechu]

I also would request ability to remove keys on that screen as part of this feature request.
Since I created a bunch of garbage keys while testing this issue: #422.