Improve instance handling / login

[micahmo]

Pull Request Description

This PR utilizes the new instance list in two ways.

1. Check the list when determining whether a potential user/community link refers to a real Lemmy server for faster handling (we still have our slow API check method as a backup).
2. Autocomplete instances when logging in.
   • Note that I did not prepopulate a default instance nor even showed the list as a dropdown. The list only appears to assist with autocomplete once the user has started typing. I did this to try to avoid pushing anyone to any particular instance (like the first in the list).

In addition to the above, I've added some links to the login page.

• When no instance is selected, a Getting Started link will navigate to join-lemmy.org.
• When an instance is selected, there's a link to open that instance in the browser, or go that instance's sign up page to make an account.

Review without whitespace.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

qemu-system-x86_64_DGojNgg27U.mp4

Checklist

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

[ggichure]

LGTM

[hjiangsu]

LGTM!