BibEdit: several bugfixes

• Avoids crashing when pending_changes holding pen related list is empty.

• Does not allow a record to be opened in two different windows, avoiding backend crashes.

• Fixes new record creation.

• Fixes locking when record opened in two different editors.

• Fixes bug on template merge that was removing some content from cache.

• Saves changes before creating a new record or cloning.

• Fixes problems when clicking back button in browser.

• Fixes textmarc2xml conversion with empty textarea.

• Fixes merge template while browsing history.

• Avoids simultaneous requests.

Co-authored-by: Alessio Deiana <alessio.deiana@cern.ch>
Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>