bug 1909: add delete field/subfield button to MARC editor

bug 1909: add delete field/subfield button to MARC editor

A button to delete (non-mandatory) subfields and fields
is added to the bib and authority MARC editors.  This button,
which displays as a hypen or minus sign after the plus sign
to clone a field, acts as follows:

[1] When deleting a whole MARC field, if at least
    one other repeat of that field exists, deletes
    the field from the editor page.  If the field
    to be deleted is the last instance of that tag,
    the contents of the field are cleared, not removed.
    This allows one to delete all 650 tags, then
    add a new one without having to reload the
    record in the editor.

[2] When deleting a subfield, if at least one other
    repeat of that subfield and its tag occurs
    *anywhere else in the record, not necessarily in the same tag*,
    deletes the subfield.
    Otherwise, if the subfield is the last occurrence
    of that tag/subfield combination, clears the input
    form instead.

Documentation note: new screenshots for MARC editor, plus
description of the '-' button.

Credit to MJ Ray for introducing the '-' button
and the UnCloneField JavaScript function.

Signed-off-by: Andrew Moore <andrew.moore@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>