Add svg linter and fix incorrect svgs (#30086)

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">

1 files changed, 26 insertions, 0 deletions

diff --git a/tools/lint-templates-svg.js b/tools/lint-templates-svg.js
new file mode 100755
index 0000000000..72f756400d
--- /dev/null
+++ b/tools/lint-templates-svg.js
@@ -0,0 +1,26 @@
+#!/usr/bin/env node
+import {readdirSync, readFileSync} from 'node:fs';
+import {parse, relative} from 'node:path';
+import {fileURLToPath} from 'node:url';
+import {exit} from 'node:process';
+import fastGlob from 'fast-glob';
+
+const knownSvgs = new Set();
+for (const file of readdirSync(new URL('../public/assets/img/svg', import.meta.url))) {
+  knownSvgs.add(parse(file).name);
+}
+
+const rootPath = fileURLToPath(new URL('..', import.meta.url));
+let hadErrors = false;
+
+for (const file of fastGlob.sync(fileURLToPath(new URL('../templates/**/*.tmpl', import.meta.url)))) {
+  const content = readFileSync(file, 'utf8');
+  for (const [_, name] of content.matchAll(/svg ["'`]([^"'`]+)["'`]/g)) {
+    if (!knownSvgs.has(name)) {
+      console.info(`SVG "${name}" not found, used in ${relative(rootPath, file)}`);
+      hadErrors = true;
+    }
+  }
+}
+
+exit(hadErrors ? 1 : 0);