This drops fomantic-ui as css toolkit and introduces tailwindcss. With
tailwind there are no predefined components, but it's very easy to
create those. So customizing the look&feel is much simpler, most of
the time no additional css is needed.
This requires a complete rewrite of the markup + styles. Luckily all
logic can be kept as is. The now old ui is not removed, it is still
available by using a request header `Docspell-Ui` with a value of `1`
for the old ui and `2` for the new ui.
Another addition is "dev mode", where docspell serves assets with a
no-cache header, to disable browser caching. This makes developing a
lot easier.
Replaced semantic-ui with the drop-in replacement fomantic-ui [0]
which is a maintained fork. The fomantic-ui used here is a custom
build [1] of the less-version _without_ google-fonts (css-only). The
javascript part of fomantic-ui is not used, and also jquery could be
dropped now.
[0] https://fomantic-ui.com
[1] https://github.com/eikek/fomantic-slim-default
Issue: #349
