Creating a service pack

This is relevant for maintaining the repository only, regular users will not need this guide.

The tasks below are in a format that's convenient for copying/pasting into a gitlab issue, where each task is checkbox.

Finish up the current service pack:
- after merging patches into pmaports master or aports master, that we want to have in the service pack:
    either just immediately cherry-pick it to the branch (no merge request and not much code review, we'll review before releasing it), or add it as TODO checkbox to the service pack discussion issue
- [ ] update the version number in the rootfs-etc-os-release file in the postmarketos-base package to add/update the service pack number after the regular branch version
- [ ] shortly (~1 week) before releasing the service pack, create the merge request
    - [ ] add the following labels:
        - release-vYY.MM
        - backport
    - [ ] enable the checkbox to delete the branch after merge
after merging the service pack:
    - [ ] homepage:
        - [ ] publish a blog post with a short overview of changes
             - [ ] add one good photo at the top (when in doubt ask Martijn)
             - [ ] one bullet point with exact version and related link (usually changelog) for updated components
             - as usually, create a merge request, make sure the team sees and reviews it
        - (do _not_ change latest_release/latest_release_title in config/download.py!)

Prepare the next service pack (right after releasing the current SP):
- [ ] create a branch for the next service pack (like v21.03.1)
- [ ] create a discussion issue for the next service pack, where we can write down if something is missing or if something in that branch is debatable
   - [ ] add the ETA calendar week (e.g. CW 10, usually 2nd CW of the month, when in doubt ask ollie)
   - [ ] add the following labels:
        - release-vYY.MM
        - service-pack-discussion
   - [ ] paste the current version of this todo list [from the wiki](https://wiki.postmarketos.org/wiki/Creating_a_service_pack) into this new issue, as separate comment
- [ ] leave a comment in the current service pack discussion issue that points to the next one
- [ ] close the old service pack discussion issue
- [ ] update [releases](https://wiki.postmarketos.org/wiki/Releases) wiki page (move the old one to released, add the new one to planned)