Now defaults to 16 on both frontend and backend.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Introduce 'make svg' which calls a node script that compiles svg files
to `public/img/svg`. These files are vendored to not create a dependency
on Node for the backend build.

On the frontend side, configure webpack using `raw-loader` so SVGs can
be imported as string.

Also moved our existing SVGs to web_src/svg for consistency.

Fixes: https://github.com/go-gitea/gitea/issues/11618

This changes the links on headings like '# Usage' in markdown from

  `https://host/user/repo#user-content-usage`

to just

  `https://host/user/repo#usage`

matching GitHub and GitLab. The linked id elements still have the prefix
and this behaviour matches GitHub and GitLab too, so JS is needed to
scroll to the active anchor. I suspect it's like that to avoid namespace
collission between user-generated content and other page content.
Compatibilty for old links is included so they will continue to work.

Also included are some enhancements to make the clickable area for the
link icon larger and fix its color on arc-green.

Fixes: https://github.com/go-gitea/gitea/issues/11896
Fixes: https://github.com/go-gitea/gitea/issues/12062