Expose 'indico' pytest plugin

That way plugins can get the same pytest environment
Update test dependencies
Add pygments/sqlparse to dev requirements (for db logs)
Support indico plugin loading in unittests

Done by adding 'indico_plugins = ...' to pytest.ini
Remove console log
Add plugcord icon
Plugins list - new layout
Implement new forms layout
Add fix for buttons in Firefox
Post-WIP changes
Change plugin icon to puzzle
Post-WIP changes
Create new radio button widget
Plugins related styling changes
Add delete icon - adding new row i-table-widget
Fix layout of special-meaning buttons

The right border color of the last button in a toolbar was always
set to gray.
Add file-content icon
CSS for tables with collapsable rows
Add selection tag in i-form
Add plus button