Admin tools

This PR refactors various tools used for admin repo checking:

- `scc check-milestone` now supports recursively explores submodules 
- `scc check-milestone` now takes the release number as the first positional argument and constructs the prefixed tag per repository using `get_tag_prefix`, e.g.

  ```
sebastien@jrs-macbookpro-25399:openmicroscopy (dev_5_0) $ python ~/code/ome/snoopycrimecop/scc/main.py check-milestone 5.1.0-m0 origin/develop
Repository: openmicroscopy/openmicroscopy
No milestone for PR 2026: Fix Memoizer injection (Fix #11934) (rebased onto develop)
Repository: openmicroscopy/bioformats
No milestone for PR 840: SDT: read each data block into a separate series
No milestone for PR 861: xsd-fu template documentation and metadatastore bugfixing
No milestone for PR 870: Update CI build names to point at the latest Bio-Formats jobs (rebased onto develop)
No milestone for PR 868: Make sure that the configuration file suffix is propagated to subdirs (rebased onto develop)
No milestone for PR 865: Remove docs/release section from the Bio-Formats repository (rebased onto develop)
No milestone for PR 853: Ignore text nodes when parsing XMLAnnotation's value
Repository: ome/scripts
No milestone for PR 62: Use fromstring() instead of fromarray() #11912
Repository: openmicroscopy/ome-documentation
No milestone for PR 637: Adding OMERO 5.1 jobs to CI doc
sebastien@jrs-macbookpro-25399:openmicroscopy (dev_5_0) $ 
```
- basic checks for repository push permissions are implemented when the `--set` option is passed to `scc check-milestone` and `scc check-labels`
- `scc unrebased-prs` is renamed as `scc check-prs` and the corresponding unit/integration tests are renamed