Implement MultiCertifyActivity

[Valodim]

Right now, our CertifyKeyActivity works for exactly one key. However, for "key signing party" style key signing, a way to sign multiple keys all in one go would be helpful. There are several different ways to go about this, opening this issue for discussion.

The minimum functionality we need is display of a given list of keys and subsequent certification. A more general dataflow of the activity is to create a list of keys, provide the user with enough info to decide on which to actually certify, and ultimately certify them all - the granularities of each of these steps are not trivial decisions.

The keys can be acquired by different means, the most important of which I would think being a previous safeslinger exchange (ping #638), or to a lesser degree manual accumulation by a number of nfc or qr actions.

[dschuermann]

an idea for uncluttering the UI: group user ids with the same name under one header