Bug 29066: (follow-up) Use tooltip when SearchMyLibraryFirst in effect

Bug 29066: (follow-up) Use tooltip when SearchMyLibraryFirst in effect

This patch proposes to add a Bootstrap tooltip to the OPAC masthead's
search field to indicate to the user when SearchMyLibraryFirst is in
effect. I think this lets the form layout stay clean-looking and is more
informative to the user because they don't have to look for the HTML
title tag tooltip.

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

- Go to Administration -> Global system preferences.
- Set the OpacAddMastheadLibraryPulldown preference to "Don't add."
- Set the SearchMyLibraryFirst preference to "Limit."
- Go to the OPAC and confirm that the main search field in the masthead
  has a filter icon in the background, indicating that searches will be
  limited.
- Hover your mouse over the search field and a tooltip should pop up,
  "Searching in <library> only"
- The message should also be triggered when you click inside the form
  field.
- Change the OpacAddMastheadLibraryPulldown and SearchMyLibraryFirst
  preferences again and the form should return to its normal behavior.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>