gerrit.py: Handle plain change number in GerritSession.change_number_from_url

This is a followup to !437 (merged).

plugins/gerrit.py:

• GerritSession.change_number_from_url now accepts a plain change
  number string.
• GerritSession.change_number_from_url now accepts only ASCII digits
  in change numbers.

tests/scap/plugins/test_gerrit.py:

• Updated TestGerritSession accordingly.

plugins/backport.py:

• Move the initialization of change_numbers closer to similar code.
• Backport._change_number: Remove logic that's now in
  GerritSession.change_number_from_url.