Fix opengraph meta for wiki pages (#4427)

Fixes https://codeberg.org/forgejo/forgejo/issues/4417 by adding a conditional branch to the `head_opengraph` template to match wiki pages. I tried to be consistent with the other types:

- `og:title` is the wiki page title
- `og:url` is built via `{{AppUrl}}{{.Link}}` like it is done for commit and file views. This has the caveat of doubling the slash (see test below). Should we `{{trimSuffix "/" AppUrl}}` to remove this, if sprig is available?
- `og:description` is the repository description to match GH behaviour. Also, the first sentences of the page might not be descriptive enough. Should we prefix the repo description with the repo name?
- `og:type` and `og:image` are common

Added a `TestOpenGraphProperties` integration test using existing fixtures. Coverage is not 100% but can be improved later.

## Output on a test repo

```html
<meta property="og:title" content="Project architecture">
<meta property="og:url" content="http://localhost:3000//xvello/wiki-test/wiki/Project-architecture">
<meta property="og:description" content="description for a test project">
<meta property="og:type" content="object">
<meta property="og:image" content="http://localhost:3000/avatars/3dd4d1e4eef065d1b4ad4bdb081ab6e7">
```

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4427
Co-authored-by: Xavier Vello <xavier.vello@gmail.com>
Co-committed-by: Xavier Vello <xavier.vello@gmail.com>

templates/base/head_opengraph.tmpl

@@ -24,6 +24,12 @@
 				<meta property="og:description" content="{{StringUtils.EllipsisString $commitMessageBody 300}}">
 			{{- end -}}
 		{{end}}
+	{{else if .Pages}}
+		<meta property="og:title" content="{{.Title}}">
+		<meta property="og:url" content="{{AppUrl}}{{.Link}}">
+		{{if .Repository.Description}}
+			<meta property="og:description" content="{{StringUtils.EllipsisString .Repository.Description 300}}">
+		{{end}}
 	{{else}}
 		<meta property="og:title" content="{{.Repository.Name}}">
 		<meta property="og:url" content="{{.Repository.HTMLURL}}">