Display text changes when a post had been edited.

**Is your feature request related to a problem? Please describe.**
When a user edits a post, Phanpy can display the earlier versions of the post. But the changes between the versions are not highlighted. This makes it very difficult to find small changes in larger posts.

**Describe the solution you'd like**
Added and removed text should be marked. Underlining and strike-through might be enough (and better than different colours for people with view problems). An explicit note “Only whitespace changes” might also be useful.