add mod respawn

[bgstack15]

This adds a new feature: support mod respawn places database.

The admin can configure the color of the respawn "places." There are a few other data in mod respawn, which might be implemented on the map in the future, e.g., per-team spawn points, which would benefit from additional color settings.

This bundles in the commits from #468 which is about Mineclonia signs, but if necessary, that can be extracted.

AI disclosure: approximately 10% of this code was written by AI:

• mcl_sign.go decodeUText()
• RespawnOverlay.js escapeHtml()

[BuckarooBanzay]

You can ignore the jshint issues, the linter/hinter is outdated from the looks of it:

map/overlays/RespawnOverlay.js: line 13, col 20, 'async functions' is only available in ES8 (use 'esversion: 8').
map/overlays/RespawnOverlay.js: line 56, col 7, Missing semicolon.
map/overlays/RespawnOverlay.js: line 79, col 11, 'async functions' is only available in ES8 (use 'esversion: 8').

AI disclosure: approximately 10% of this code was written by AI:

Nothing wrong with that, if it gets the job done 🤷

Also: thanks for the caching-fixes in the Dockerfile 👍

[bgstack15]

I guess I overlooked that I had manipulated the docker file. That was for my own benefit while iterating, but it probably would help anybody else trying to build.