Add instance page

[micahmo]

Pull Request Description

This PR introduces a very simple instance page. Currently it is only accessible from the post long-press menu and essentially only displays the instance heading and sidebar. It does also have the option to open the instance in the browser.

This should provide a groundwork for many other things such as...

• a place to block an instance
• the ability to open this page from many other places (e.g., account switcher)
• seeing a list of communities on an instance

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

qemu-system-x86_64_3Q2eV6vMik.mp4

Checklist

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

[hjiangsu]

Good job overall, I just had a few comments/feedback on some minor points!