Use loading page for posts/comments

[micahmo]

Pull Request Description

This PR adds the loading page feature for post and comment links. The loading page was originally added in #1200, and it was to handle the case where we have an ambiguous link for a person or community, so we load the full object to verify whether it really exists before navigating. The loading page helps to bridge the gap while we fetch the object so the user knows something is happening.

I've been noticing that navigating to posts and comments has a similar problem where there seems to be no activity for a period of time (especially on a slower network). I believe this is because we similarly load the whole post/comment object before navigating, and unlike users/communities, we always load posts/comments, making the problem even more obvious. Furthermore, we often have to do a ResolveObject if the entity is on a different instance.

Review without whitespace.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Before

post-comment-loading-before.mp4

After

post-comment-loading-after.mp4

Checklist

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

[hjiangsu]

LGTM! Just one comment!