[FEAT] Adds x-mode-only anchor styles to display images based if the theme is light or dark mode. (#3985)

Adds a feature similar to this https://github.blog/changelog/2021-11-24-specify-theme-context-for-images-in-markdown/ , by adding styles to elements which `src` or `href` attribute ends with `#light-mode-only` or `#dark-mode-only`. To improve compability, the github variants with the `gh-` prefix are also contained.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3985
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Mai-Lapyst <mai-lapyst@noreply.codeberg.org>
Co-committed-by: Mai-Lapyst <mai-lapyst@noreply.codeberg.org>

models/fixtures/comment.yml

@@ -83,3 +83,12 @@
   issue_id: 2 # in repo_id 1
   review_id: 20
   created_unix: 946684810
+
+-
+  id: 10
+  type: 0
+  poster_id: 1
+  issue_id: 1 # in repo_id 1
+  content: "test markup light/dark-mode-only ![GitHub-Mark-Light](https://user-images.githubusercontent.com/3369400/139447912-e0f43f33-6d9f-45f8-be46-2df5bbc91289.png#gh-dark-mode-only)![GitHub-Mark-Dark](https://user-images.githubusercontent.com/3369400/139448065-39a229ba-4b06-434b-bc67-616e2ed80c8f.png#gh-light-mode-only)"
+  created_unix: 946684813
+  updated_unix: 946684813

models/fixtures/issue.yml

@@ -10,7 +10,7 @@
   priority: 0
   is_closed: false
   is_pull: false
-  num_comments: 2
+  num_comments: 3
   created_unix: 946684800
   updated_unix: 978307200
   is_locked: false