Bug 28394: Improve style of patron category entry form

Bug 28394: Improve style of patron category entry form

This patch makes some style and structure changes to the patron
cartegory entry form:

- Margin added to the enrollment period fieldset so that it aligns with
  the rest of the form.
- Informational text inside the form wrapped in <div class="hint"> like
  it is on other pages.
- Converted heading tag to legend tag in the messaging preferences
  fieldset.

To test, apply the patch and rebuild the staff interface CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Go to Administration -> Patron categories -> New category and confirm
that the style updates look good.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>