Replace tribute with text-expander-element for textarea (#23985)

The completion popup now behaves now much more as expected than before
for the raw textarea:
- You can press <kbd>Tab</kbd> or <kbd>Enter</kbd> once the completion
popup is open to accept the selected item
- The menu does not close automatically when moving the cursor
- When you delete text, previously correct suggestions are shown again
- If you delete all text until the opening char (`@` or `:`) after
applying a suggestion, the popup reappears again
- Menu UI has been improved

<img width="278" alt="Screenshot 2023-04-07 at 19 43 42"
src="https://user-images.githubusercontent.com/115237/230653601-d6517b9f-0988-445e-aa57-5ebfaf5039f3.png">

templates/shared/combomarkdowneditor.tmpl

@@ -39,7 +39,9 @@ Template Attributes:
 				<span class="markdown-toolbar-button markdown-switch-easymde">{{svg "octicon-arrow-switch"}}</span>
 			</div>
 		</markdown-toolbar>
-		<textarea class="markdown-text-editor js-quick-submit" name="{{.TextareaName}}" placeholder="{{.TextareaPlaceholder}}">{{.TextareaContent}}</textarea>
+		<text-expander keys=": @">
+			<textarea class="markdown-text-editor js-quick-submit" name="{{.TextareaName}}" placeholder="{{.TextareaPlaceholder}}">{{.TextareaContent}}</textarea>
+		</text-expander>
 	</div>
 	<div class="ui tab markup" data-tab-panel="markdown-previewer">
 		{{.locale.Tr "loading"}}