Improve UI template customization feature

• The default location for custom templates is /var/lib/debusine/server/templates. Neither Enrico nor Helmut understands why that location is preferred over something below /etc. Can we change that default to /etc/debusine/server/templates or additionally search that location? The key distinction here is that /var is typically meant to be modifiable by the service, but templates likely should not be modifiable. (!2112 (merged))
• /usr/lib/python3/dist-packages/debusine/project/checks.py function _check_directories_owners rejects templates owned by the root user and makes debusine-admin check (and as a consequence debusine-server.service) fail. It should accept root-owned files. (!2050 (merged))
• The documentation should mention that the template cache cannot be cleared without restarting debusine-server.service. (!1976 (merged))