Prepare torbrowser-launcher release 0.3.7

The current master contains an important fix for the TBB download URL that got worked around by server-side redirect rules, but since I suppose that that the workaround is just a temporary solution, it would be a good idea to fix this properly by tagging a new stable torbrowser-launcher release. There are also some AppArmor fixes since the last stable release, but nothing that could break things and would require extensive testing.

Since I have no experience with new torbrowser-launcher releases, It would be great if @micahflee could do the release. But feel free to ask me for any help if needed. :-)

Before releasing the new version, it would be great if this MR could get merged. Also, make sure that the version number/date gets bumped in the AppStream metadata file to reflect the new release as documented here.