User navigation fix and error handling

[micahmo]

Pull Request Description

This PR has a couple of fixes for issues related to navigation.

First, I noticed that our regexes for communities were accidentally matching a particular username syntax, like this: /u/[email protected], because we were essentially looking for anything@anything. With this fix, if the community checker parses a name starting with /u/, it will be ignored.

Secondly, I added some error handling around the community navigation page. While we're now less likely to get into this error scenario, it would still be nice to know if the navigation is failing because we are trying to load a community we can't find, so now there is a snackbar indicating when a community load fails.

Review without whitespace.

Issue Being Fixed

Issue Number: Could possibly be related to #1006.

Screenshots / Recordings

This video demonstrates the new error handling prior to fixing the community detection. This would clue us in that we're trying to load this user as a community, and something else is wrong to be investigated!

nav1.mp4

This video demonstrates the correct user navigation after applying the community detection fix.

nav2.mp4

Checklist

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

[hjiangsu]

I just noticed one issue with this as I was testing it - the drawer icon no longer opens the feed/subscriptions drawer.

This seems to be related to the Scaffold that was added in feed_page.dart. When tapping on the drawer icon, it attempts to look for the closest Scaffold parent in the widget tree which happens to be the one that was added in. Since that Scaffold does not have a drawer associated with it, it doesn't perform any action!