Build info endpoint

[rymiel]

Closes #16. Closes #109.

Adds the endpoint /api/build, which returns

• server start time,
• adventure-text-minimessage version,
• webui commit hash at build time, and
• configured bytebin URL (https://bytebin.lucko.me by default)

These are mostly set through application.conf. The client will check this build endpoint to determine what bytebin instance to use, then cache it for next time