[WIp] Automated nightly Howl Flatpak builds

[refi64]

Basically, it is what it says. You can try out the repo:

# Add the remote
flatpak remote-add --user --no-gpg-verify howl-nightly https://storage.googleapis.com/howl-dl/repo
# List the refs inside
flatpak remote-ls howl-nightly
# Install Howl nightly
flatpak install howl-nightly io.howl.Editor

Why is this [WIP]?

There are four more things that probably need to be done:

• Right now the cloud bucket bucket name is just howl-dl. If we want to use a shorter URL, e.g. off the howl.io domain (something like dl.howl.io), it needs to be changed here to match.
• Dropping --no-gpg-verify is generally a good thing, but first the commits need to be GPG-signed. In general using a new, separate key from anyone's normal one is preferred, but I wasn't sure if @nilnor would prefer it point to e.g. his email or mine or something else, so I didn't create one yet.
• Related to the above, we can create a .flatpakrepo file that a user can double-click to automatically add the remote, but it depends on GPG signatures being enabled, so see the above point.
• Make artifacts public (see below).

In addition to the scheduled "nightly" builds (I haven't actually enabled the trigger yet), Azure Pipelines will build a Flatpak bundle for every PR or commit. So, if someone wants to try something out, they can just download the Flatpak bundle and install it to give that PR / commit / whatever a spin. You can see the current bundles here, just select a build, then go to Artifacts -> Flatpak bundle -> download howl.flatpak. These artifacts need to be published in a slightly different way to make them public but I haven't done that yet.

So yeah, initial feedback welcome. 😁

EDIT: Also the commit history is a mess, please squash when merging.