Fix watchers-list in details pane

- fixed watchers-list in details pane
- changed color of the remove icon from black to red
- added global color variables
Living Style Guide: Document OpenProject colors

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1954 from opf/fix/watcher-overlapping-in-details-pane

Fix watchers-list in details pane