Bug 9894 - Followup: Add support for "flagged" values

Bug 9894 - Followup: Add support for "flagged" values

Even though we don't know what they mean or do.

To test:
- open the docs to have at hand the list of codes/visibility options combinations and
try changing several subfields from a single tagfield. Save, and reopen the "Edit subfields" page.
- Click the "Flagged" checkbox - it should disable the other checkboxes as this setting is (apparently) exclusive to the others.
- Step through several tabs and set your visibilities as you like.
- I had a MySQL shell opened to repeatedly run this query (i tested against field 886
  on the default framework):
>  SELECT tagfield,tagsubfield,hidden FROM marc_subfield_structure WHERE frameworkcode='' AND tagfield=886;
- Compare what was saved on the DB with the docs for that visibility selection.

Bug 9894 - Help file updates

To test:

Click the Help link in the upper right hand corner of the MARC subfield editor
Verify that the epic list of values is gone
Verify that the help page overall looks alright
Verify that the help makes sense and is understandable.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>