Bug 1859 - Notice fields: can't select multiple fields at once

Bug 1859 - Notice fields: can't select multiple fields at once

When selecting database fields to appear in a notice, selecting multiple
fields at once doesn't work correctly. The selected fields are grouped
together between << and >> instead of each being bracketed separately.

This patch corrects this problem. Also changed: Removal of 'onclick';
Conversion of 'insert' button to a Bootstrap buttons styled with a Font
Awesome icon; HTML validation error fixes.

To test, apply the patch and go to Tools -> Notices & Slips.

- Edit any notice.
- Select a notice version to edit (Email, Print, etc.).
- Confirm that the "Insert" button looks correct.
- Using ctrl+click, select multiple database fields to add to the
  notice. Confirm that clicking the "Insert" button adds the fields
  correctly.
- Confirm that there are no HTML validation errors.

Signed-off-by: Nicole C Engard <nengard@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>