Signed-off-by: Anirudh Oppiliappan <anirudh@tangled.sh>

- Disable caching of responses as suggested by documentation.
<https://git-scm.com/docs/http-protocol#_smart_server_response>
- Return plain-text errors so the git client will display them.
- Check Content-Type of upload-pack request.

v1-style requests are unaffected and v0 (dumb-protocol) requests remain
unsupported.

this gets noisy really quickly when the stack is rebased. now, the only
things that can cause a resubmit are: reauthoring commit messages or
modifying the patch itself.

speak more about tangled itself, link to discord, irc and source code.

- fix kotlinscript detection (there is an open patch upstream for this)
- fix gleam highlighting

the stacking working introduced some unnecessary changes here