Closes #593

This implements a generic `TrustIndicatorView` with a `setTrustLevel()`
method which updates the drawable according to the `Author.State`.

Closes #78

This commit renames identicons to avatars in field names, and uses an ImageView
derivative to show the identicons, which should make implementing #214 easier.

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.