REGISTRAR,GUI: Support for Heading and Timezone in app form

- Added new form item HEADING, which will be used as customizable
  heading of application form. Content is considered as HTML.

- HEADING is not saved with submitted application form.

- HTML_COMMENT and HEADING now have customized language pages
  on app form settings, since they only contain label and not
  error or help text.

- Added new form item TIMEZONE, which is pre-filled with
  existing timezones in GUI and user can manually select
  one or select "Not selected" option. When required, user
  must select one of timezones.
  Form item still must be manually linked to proper attribute,
  which is urn:perun:user:attribute-def:def:timezone.

- Fixed "onChange" behavior of all selection boxes on application
  form, since their validation was previously performed only on
  form submission. It's now performed on any selection change.