Bug 35919: Add Koha::Biblio::Metadata->source_allows_editing

Bug 35919: Add Koha::Biblio::Metadata->source_allows_editing

This patch adds a convenient method that tells if the record is
editable, according to it's record source.

To test:
1. Apply this patch
2. Run:
   $ ktd
  k$ prove t/db_dependent/Koha/Biblio/Metadata.t
=> SUCCESS: Tests pass! Use cases covered!
3. Sign off :-D

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Bug 31791: Rename can_be_edited -> source_allows_editing

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>