Message Editing: Display edits as diffs

[jryans]

We'd like to display edit history as a diff to make it obvious what was added / removed / changed.

However, we should only do this if we can be absolutely certain that we know the immediate predecessor of an edit without gaps, etc.

• diff text messages
• diff html messages

[bwindels]

Consider using https://github.com/google/diff-match-patch , as riotx (not for html messages though) does