Bug 15880 - Serials new frequency link should be a toolbar button

Bug 15880 - Serials new frequency link should be a toolbar button

This patch converts the "new frequency" link to a Bootstrap-style
toolbar button in order to maintain consistency. Also included in this
patch are some other minor markup corrections:

- A colon and space after labels
- A 'Cancel' link instead of a button.
- "Required" classes for a required field.

To test, apply the patch and go to Serials -> Manage frequencies.

- Test the "New frequency" button
- Confirm that the changes to the entry form look correct.

Followed test plan, displays as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com