conversion service: use pathlib
flash messages: enclose in a grid row
* abilian/web/assets/filters.py: don't fix url path if it's embedded base64 data
fix grid markup
* abilian/web/resources/less/form.less: select inputs: unify look accross browsers
* abilian/web/resources/less/layout.less: #content: use @grid-gutter-width for margin / padding