BibMerge: subfield diff/edit, use new BibEdit API
BibMerge: subfield diff/edit, use new BibEdit API

* BibMerge: Add, replace, delete subfield implemented

* BibMerge: Added diff button for subfields

* BibMerge: Updated bibmerge_engine.py to new bibedit_utils API

* BibEdit: Fixed error messages wording.

* BibEdit: Disabled non-supported links to BibMerge.

inveniosoftware/invenio-base,tiborsimko/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base