invenio.cfg: Move bibtex formatters together in dropdown

[sakshamarora1]

No description provided.

[ntarocco]

I would prefer that instead of adding **RECORD_EXPORTERS, we probably do:

APP_RDM_RECORD_EXPORTERS = {
  RECORD_EXPORTERS['first'],
  RECORD_EXPORTERS['...'],
  ....

WDYT?
Ping @kpsherva @zzacharo

[ntarocco]

Actually, @sakshamarora1 can you simply sort the keys and that's it? I think that it makes sense to me to have them alphabetically sorted.