Rewrite xapian-bridge in C

This commit ports xapian-bridge from GJS to C. The port is meant as a
drop-in replacement for the former code, with 1:1 compatibility.
Convert tests to gtest

Convert our unit tests to C tests using GLib's gtest.
Remove eos-jasmine submodule

Now that our tests are in C, we don't need this anymore.
Move systemd files to main branch

No need for these to live in the debian branch. They're useful on
non-debian systems as well.
Update .gitignore