Add top-level comment navigation

[zandm7]

Describe the feature

Add buttons in thread view that allow the user to jump between top-level comments.

Additionally, a toggle in settings to allow the user to jump between comments with the volume buttons would also be really nice.

Additional context

See the following for more details:

• Thunder for Lemmy (https://github.com/thunder-app/thunder/blob/develop/lib/shared/comment_navigator_fab.dart)
• Summit for Lemmy (https://github.com/idunnololz/summit)
• Boost for Lemmy/Reddit (https://play.google.com/store/apps/details?id=com.rubenmayayo.lemmy&hl=en-US)