Highlight with Prism in a worker and gzip text responses

Highlight with Prism in a worker and gzip text responses

The code browser now names a blob's language and lets the browser colour it.
Chroma is gone: server/web/highlight.go is an extension table, and
static/prism-worker.js runs Prism — every language Prism ships — off the main
thread. The server sends escaped lines with their numbers and L# anchors, so a
page is complete and scrollable before any script runs and the worker only swaps
the contents of each line cell. The table covers 186 languages; a file it does
not claim stays plain rather than being coloured by a guess.

The bundle those grammars make is 545 KB, so the instance now gzips text
responses itself instead of relying on a proxy, and static assets carry a
content ETag so a browser revalidates with a 304 rather than re-downloading the
bundle on every code page. The raw-download route opts out with
Cache-Control: no-transform because it promises an exact Content-Length.

This also makes JavaScript a requirement rather than a nicety: the label and
assignee token inputs lose their noscript save buttons and their comma-separated
text fallback, and the docs say so.

Measured: release binary 26.08 MB -> 22.70 MB; /r/{repo}/code 11.1 KB -> 2.6 KB
over the wire; the Prism bundle 545 KB -> 206 KB gzipped, fetched once.

scripts/vendor-prism.py regenerates the bundle from Prism's components.json
require graph, and --check fails if the committed assets drift.