LP#1765501: ensure that webstaff MARC editor can deal with short 008s

LP#1765501: ensure that webstaff MARC editor can deal with short 008s

This patch ensures that the 008 field, when maintained by the web staff
MARC editor, is kept at its proper length. In particular, this solves a
problem where input can get messed up if the user selects the entire
contents of the 008 field and replaces it with a shorter string.

To test
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[1] Create a new bib record. Ensure that the MARC record editor is
    /not/ in flat text mode.
[2] Edit the 008 field by selecting its entire contents and replacing it
    with a subset. For example, this might be done when the first action
    that a cataloger takes is to set the 008/00-05.
[3] Enter value in the individual fixed-field inputs at the top of the
    MARC editor form. For example, enter 'eng' in the Lang field.
[4] Note that the results are wrong; e.g., the text ends up as
    "080419en en eng".
[5] Apply the patch and repeat steps #1-#4. This time, the results
    should be correct.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Cesar Velez <cesar.velez@equinoxinitiative.org>