Fixes https://github.com/go-gitea/gitea/issues/30082.
Adds a new linter that searches for non-existant SVG images in
templates. Output before the fix was:
```
$ make lint-templates
SVG "octicon-warning" not found, used in templates/devtest/flex-list.tmpl
SVG "octicon-warning" not found, used in templates/devtest/flex-list.tmpl
SVG "octicon-markup" not found, used in templates/repo/diff/comment_form.tmpl
make: *** [Makefile:438: lint-templates] Error 1
```
<img width="306" alt="Screenshot 2024-03-25 at 23 31 05"
src="https://github.com/go-gitea/gitea/assets/115237/1052d1a9-bfec-4d5a-9cae-f895f78f7c93">
silverwind