Bug 21740: Show @ when editing fixed-length fields on bibliographic frameworks

Bug 21740: Show @ when editing fixed-length fields on bibliographic frameworks

This is a consistency issue: we store '@' as the subfield for
fixed-lenght fields, yet on editing '_' is displayed (on the form and
the tab). This was probably done to avoid URI encoding problems in the
past.

This patch restores the '@' everywhere.

To test:
- Open the MARC structure for any bibliographic MARC framework
- Choose Actions > Subfields on any fixed-length field
=> ERROR: Notice the tab has an underscore (_) and the subfield code is '_' as well.
- Cancel
=> SUCCESS: '@' is displayed on the list
- Apply this patch
- Restart all the things:
  $ restart_all
- Choose Actions > Subfields on any fixed-length field
=> SUCCESS: '@' is displayed in the tab title, and on the 'Subfield
code' form field.
- Save some changes
=> SUCCESS: Things work correctly.
- Sign off :-D

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Devinim <kohadevinim@devinim.com.tr>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>