Refactor: reorganization of files to their domain

[hjiangsu]

Pull Request Description

This is yet another refactor PR. This PR mainly focuses on moving some files to their respective domains. This aim is to make the code structure a bit easier to navigate (and reduce the number of files store in utils directory). Let me know what you think of this!

In general:

• Moved utils/comment.dart and utils/navigate_comment to comment/utils
• Moved utils/navigate_instance to instance/utils
• Moved utils/navigate_create_post and utils/navigate_post to post/utils
• Moved utils/navigate_user to user/utils

In addition:

• shared/hero was removed since its no longer being used anywhere
• Added check for FlutterDisplayMode to only trigger on Android. This removes an error when first opening Thunder on iOS

Sidenote: I'll be doing this in stages, so that it's easier to manage. I would like to do this for other widgets (especially ones that are in the shared directory such as the comment_ related widgets)

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Very nice, I like the structure!