[Rework] WebUI: drop Font Awesome framework

[Rework] WebUI: drop Font Awesome framework

Replace Font Awesome's "SVG with JS" framework with a local subset SVG
sprite. Delete fontawesome.min.js and solid.min.js (~900 KB of JS) and
svg-with-js.min.css, and drop the fontawesome/fontawesome_solid RequireJS
paths and shim.

app/icons.js renders the glyphs: an initial sweep plus a MutationObserver
replace <i class="fas fa-<name>"> with inline <svg class="svg-inline--fa">
<use href="img/icons.svg#fa-<name>"/></svg>, carrying over non-class
attributes (id) and modifier classes (fa-spin, spacing). setIcon() swaps
an icon in place; the theme toggle (rspamd.js) and the map-modal header
(config.js) move off class toggling and [data-fa-i2svg] selectors. Each
icon keeps its natural aspect ratio: rspamd.css sizes the box at 1em
square by default, and icons.css (generated) overrides the width of
non-square glyphs. rspamd.css also adds the fa-spin keyframes
svg-with-js.min.css used to provide.

The sprite (img/icons.svg, 35 glyphs) and icons.css are generated by
icons-build.mjs (repo root) from icons-manifest.txt; v5 aliases (e.g.
times -> xmark) resolve at build time. Font Awesome
(@fortawesome/fontawesome-free, pinned 6.6.0) is a devDependency; CI
(ci_eslint) runs "npm run check:icons" - icons-check.mjs verifies every
fa-<name> in the markup is listed in the manifest, then rebuilds and
fails on any diff (stale files or Font Awesome version drift).