discourse: update.py: Improve version handling, use pinned plugins

Add a DiscourseVersion class which handles Discourse's version
numbering properly when sorting - beta versions are sorted lower than
their respective release versions. It can also return both its version
number and equivalent git tag, removing the need for `rev2version` and
manually adding `v` to the front.

Using DiscourseVersion instead of LooseVersion, we can list all
current version number tags from the `discourse` repo and sort them
correctly, giving us the latest one, regardless of type; i.e. we don't
have to filter for only release versions or beta versions anymore.

This also implements the plugin pinning algorithm laid out here:
https://meta.discourse.org/t/pinning-plugin-and-theme-versions-for-older-discourse-installs/156971
to make sure we don't upgrade plugins further than what's compatible
with our currently packaged Discourse version. While it likely won't
matter much most of the time if we continue packaging the beta
versions, it could be helpful if we decide to go back to packaging
release versions or if we run into issues with future upgrades. In
that case, the plugins could still be updated safely even though we're
not on the latest version of Discourse.

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