WIP: ux: Add "Follow link" button in the sidebar

Wanted to get to know the codebase a little and thought this would be a good size task (but I don't think this is on the roadmap?). I remember missing this when I used Manyverse more.

Sort of a placeholder until cypherlinks start looking like ssb:%asdf and the OS can redirect the link to the right app. This adds an item in the sidebar that brings up a prompt where you can paste the cypherlink (that you found somewhere outside of ssb, maybe in some chat app).

TODO:

• I want to refactor the link-following code (that I got from global-styles/markdown.ts) into some other file, but not sure where to put it.
• The UX could probably be a bit better, maybe a cancel button?
• I notice there aren't many awaits in the code. Should I do some stream thing instead? Guessing I should do it like src/frontend/screens/central/connections-tab/dialog.ts. OOPS I just noticed I do the link logic on all prompts, including dht invites. That's a bug. Not sure where to put that logic then. Can't put it in dialog-follow-links.ts, right, since that's mostly a definition?
• Squash the commits on merge to follow commit message convention