Bug 8064: Merge several biblios

Bug 8064: Merge several biblios

This patch improves the existing merging tool by adding possibility to
merge more than 2 biblios.
There is no functional changes:
  - Add some biblios to a list
  - In the list check some biblios and click on 'Merge selected records'
  - Choose the biblio which will be kept, all others will be deleted
  - On the next page you have all biblios you chose in tabs (left side
    of the screen) and the preview of result (right side)
  - Pick some fields or subfields from records that will be deleted or
    delete some fields from reference record.
  - Click on 'Merge', if there is no errors you are redirected to the
    biblio view.

Added checks for non-repeatable subfields
Added checks for mandatory fields and subfields before submitting the
form.
Added a final report which display deleted records (see syspref
MergeReportFields)

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>