CSS (forms) refactoring
* All forms now use the render_field helper.
* CSS is now compiled using PostCSS.
* Common form styles got extracted to more reusable & flexible classes.
Closes#12.
Co-authored-by: Wojciech Kwolek <wojciech@kwolek.xyz>
Reviewed-on: https://git.r23s.eu/wojciech/doneth.at-backend/pulls/13