Merge pdbq:introduce-mp-addresses into pdbq:main

This branch introduces a new notion of shorthand MP addresses, similar to PPA addresses and LP: #'s. So, this MP could be referenced as:

    mp:bryce/pdbq/450375

That's enough info to translate into the full URL. There are some complexities involved, which I elaborate about in the commit message. Since for now this is merely cosmetic for display purposes I'm not bothering with properly documenting it, let alone making a spec about it, but if this ends up being actually useful those would be logical actions to take next. (The right time for that action is most likely around when a parser is going to be written for it.)

Where I envision using this is in the summary listings of reviews, for both web page and command line. Currently, the value is seen when running `pkg-mp list`, which I recommend doing to test.

Example output:

* pdbq: MP #450375
  - address: mp:bryce/pdbq/450375
  - requester: bryce
  - requester_name: Bryce Harrington
  - requester_url: https://launchpad.net/~bryce
  - date_created: Thu Aug 31 20:27:21 2023
  - date_requested: Thu Aug 31 20:27:21 2023
  - date_reviewed: None
  - date_merged: None
  - source_repo: https://git.launchpad.net/pdbq/plain/?h=introduce-mp-addresses
  - source_branch: introduce-mp-addresses
  - target_repo: https://git.launchpad.net/pdbq/plain/?h=main
  - target_branch: main

* python-django-crispy-forms: MP #450376
  - address: mp:lvoytek/+python-django-crispy-forms/450376
  - requester: lvoytek
  - requester_name: Lena Voytek
  - requester_url: https://launchpad.net/~lvoytek
  - date_created: Thu Aug 31 20:55:55 2023
  - date_requested: Thu Aug 31 20:55:55 2023
  - date_reviewed: None
  - date_merged: None
  - source_repo: https://git.launchpad.net/~lvoytek/ubuntu/+source/python-django-crispy-forms/plain/?h=django4-compatibility
  - source_branch: django4-compatibility
  - target_repo: https://git.launchpad.net/ubuntu/+source/python-django-crispy-forms/plain/?h=ubuntu/devel
  - target_branch: ubuntu/devel