Ensure correct dimension.db file ownership

This is mostly here to guard against problems happening
due to server migration and doing `chown -R matrix:matrix /matrix`.

Normally, the file is owned by `1000:1000`, as expected.

If ownership changes, Dimension could still start, but it will fail the
first time it tries to write to the database. Explicitly chowning
before startup guards against this.

Related to #485 and #486 (Github Pull Requests).
Also related to ccc7aaf0ce.

roles/matrix-dimension/templates/systemd/matrix-dimension.service.j2

@@ -9,6 +9,9 @@ Type=simple
 ExecStartPre=-/usr/bin/docker kill matrix-dimension
 ExecStartPre=-/usr/bin/docker rm matrix-dimension
 
+# Fixup database ownership if it got changed somehow (during a server migration, etc.)
+ExecStartPre=-/usr/bin/chown {{ matrix_dimension_user_uid }}:{{ matrix_dimension_user_gid }} {{ matrix_dimension_base_path }}/dimension.db
+
 ExecStart=/usr/bin/docker run --rm --name matrix-dimension \
 			--log-driver=none \
 			--user={{ matrix_dimension_user_uid }}:{{ matrix_dimension_user_gid }} \