visual diffs for PRs
because you can't see the diffs as images on GitHub, it would be nice to be able to see PR in kactus

Not sure how tho. One option:
- 3rd tab (next to "Changes" and "History") "Compare"
  - you could select a branch to compare to
  - find the PRs which contain this branch and have a select box with them (or maybe a UI more similar to the "history" tab, with a first column with the list of branches to compare to (and PRs) and a second column with the changed files)
  - add a textarea to comment on a file