Remove all direct references to font-awesome (#24448)

- Related #10410
- I had to add an SVG for an empty checkbox
https://github.com/primer/octicons/issues/942

# Before

![image](https://user-images.githubusercontent.com/20454870/235374683-13f355c3-1245-40db-adda-4c710fc80288.png)

# After

![image](https://user-images.githubusercontent.com/20454870/235374655-cc637132-f314-424d-9243-13d45b8915d5.png)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>

web_src/js/features/comp/ComboMarkdownEditor.js

@@ -8,6 +8,7 @@ import {handleGlobalEnterQuickSubmit} from './QuickSubmit.js';
 import {emojiString} from '../emoji.js';
 import {renderPreviewPanelContent} from '../repo-editor.js';
 import {matchEmoji, matchMention} from '../../utils/match.js';
+import {svg} from '../../svg.js';
 
 let elementIdCounter = 0;
 
@@ -218,7 +219,7 @@ class ComboMarkdownEditor {
           cm.replaceSelection(`\n- [ ] ${cm.getSelection()}`);
           cm.focus();
         },
-        className: 'fa fa-square-o',
+        icon: svg('gitea-empty-checkbox'),
         title: 'Add Checkbox (empty)',
       },
       'gitea-checkbox-checked': {
@@ -227,7 +228,7 @@ class ComboMarkdownEditor {
           cm.replaceSelection(`\n- [x] ${cm.getSelection()}`);
           cm.focus();
         },
-        className: 'fa fa-check-square-o',
+        icon: svg('octicon-checkbox'),
         title: 'Add Checkbox (checked)',
       },
       'gitea-switch-to-textarea': {
@@ -235,7 +236,7 @@ class ComboMarkdownEditor {
           this.userPreferredEditor = 'textarea';
           this.switchToTextarea();
         },
-        className: 'fa fa-file',
+        icon: svg('octicon-file'),
         title: 'Revert to simple textarea',
       },
       'gitea-code-inline': {
@@ -249,7 +250,7 @@ class ComboMarkdownEditor {
           }
           cm.focus();
         },
-        className: 'fa fa-angle-right',
+        icon: svg('octicon-chevron-right'),
         title: 'Add Inline Code',
       }
     };