Add Flask-Migrate to enable database migrations
Also add a new "completed" column to Tasks for storing the time the task is completed.
Use flask --app taskflower db update to add the new column
- SVG icons are now preprocessed into raw HTML at first request. This allows them to be inlined (use `{{icon(icon_name)|safe}}`) and thus styled by CSS.
- General CSS improvements (especially around buttons)
- A basic role editor is now implemented. Go to `/namespace/<id>/role` to see it.
- Task and invite lists now have an "add new" button on the list page.
- Slight permission fixes
- Added `assert_left()` and `assert_right()` to `Either`s. Now, if you do `if isinstance(x, Right)`, you can `x.assert_left()` in the `else` to make the type checker happy.
