GitLab

Use a symlink-based procedure for making the update deployment atomic.
This way, at no time does the filesystem contain an invalid version of
the game and thus the webserver will never serve an inconsistent state.

To ensure we get clean version information, update the mtime of
`.git/HEAD` before running the release pipeline.  Otherwise tags would
just end up re-using the version information from the untagged commit.

Run all the web builds as well, in the pipeline for merge requests.

Cache cargo build directory (`target/`) and npm's `node_modules/`.