SPA/static support

[JeDaYoshi]

This changes how @cryb/web rendering works. As of now, it has been done using Server-Side Rendering, and since we have seen that switching to Single Page Application rendering won't hurt but more than enhance things performance-side and preventing some issues, while even allowing to host this as a static page, this will be the default from now on.

• Page actually works as it should! (No need to re-login each visit)
• Has a loading animation (ToDo: @neoncloth move styles to @cryb/borealis [which is more likely to be safe to do since styles are pre-fetched] and fix centering when on mobile.)
• Fetches user correctly, and redirects to / if it can't (as it was doing before, but now reworked to not repeat code)
• SSR still works, simple as commenting the mode: 'spa' line in nuxt.config.js to enable it back (possibly making this configurable in the future?)
• Can also use yarn generate to generate a static site, which works completely (Note: Requires rewriting rules for invites to work so it fetches /i/index.html, since those are dynamic and will 404)

Plus this, includes a few changes that I was doing before:

• Add robots.txt support (blocking everything except /, which is the only one that needs to be indexed)
• Few messages, styling and other minor changes

Seems to work fine Forgot to fix this, but other than that it works, but extra testing is appreciated.

I'm also thinking of changing some other things, but that'd be for later changes.

[JeDaYoshi]

We'd need to add later documentation so people know how to do the invite thing if they want to use a static-generated site, but it's something pretty simple and common. No extra steps to do when running Nuxt.js built-in server, though.