UX refresh

Add mobile store links

It's much more like a details / disclosure view you can expand than a typically
toggle switch.

Hide focus ring for mouse users

Fixes https://github.com/matrix-org/matrix.to/issues/120

fix: Update footer URL path to existing branch

In the footer there's a link to add clients to the tree of this
repository, which pointed to a branch that did not exist in the
repository anymore. As such GitHub rendered a 404. This change points to
the footer URL to the `main` branch, which should exist as it's the
default branch for this project.

Quick fixes: round 1

Fixes https://github.com/matrix-org/matrix.to/issues/153

Pass args to clients

Remove the existentialist crisis

Use thumbnails api instead of mxc download

Co-authored-by: Travis Ralston <travpc@gmail.com>

Fix background on large displays

Remove link around generated invite

Make identifier regexes closer to the spec