Commit the Alpine.js tarball in 3p/ and extract it at build time (closes #345)

The build no longer downloads Alpine.js. Its npm package tarball is
committed as 3p/alpinejs-3.14.9.tgz; its sha256 matches the value
script/fetch-assets pinned, and the cdn.min.js inside it matches the
value static/vendor.sha256 pinned.

make assets extracts package/dist/cdn.min.js to the ignored
static/js/alpine.min.js. make test, check, build and dev run it first,
and the Dockerfile builds through make test and make build.
script/fetch-assets, its Dockerfile step, static/vendor.sha256 and
static/vendor_test.go are removed, along with bootstrap's curl install.

Model: opus-5-5

internal/server/static_assets_test.go

@@ -13,9 +13,9 @@ import (
 
 // TestBaseTemplateScriptsAreServed walks every /s/ script the base
 // template loads on each page and fetches it through the real router.
-// Alpine.js is fetched at build time rather than committed, so nothing
-// in the repo guarantees it is present: this is the check that the page
-// still gets the JavaScript it asks for.
+// Alpine.js is extracted from its tarball in 3p/ at build time, so the
+// file is not in the tree: this is the check that the page still gets
+// the JavaScript it asks for.
 func TestBaseTemplateScriptsAreServed(t *testing.T) {
 	t.Parallel()