Bug 17874: Bug 16239 followup - polishing things

Bug 17874: Bug 16239 followup - polishing things

This patch is made on top of bug 16239 to polish things.

Makes these changes:
- Buttons on add circulation message modal
- Dropdowns has top and bottom margin 4px instead of 3px
- btn-link class (for example in toolbar on advanced search pages)
- Delete circ message is btn-link
- Add new circ message is btn-link

Test plan:
- Confirm that all changes are as described above and that they make sense

Signed-off-by: Claire Gravely <claire_gravely@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>