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

Stop weird text splitting

Future proof schemas by making them nonstrict

Update fractal description

Add rudimentary group support

Fix enter to submit behaviour

Add more sans fonts

Fixes https://github.com/matrix-org/matrix.to/issues/121