Implementation of new styling for forms and plugins' admin pages

This patch introduces:
* new CSS classes for styling forms, which can be used all over Indico,
* application of these classes in the plugins' admin forms
* new styling for the plugins list admin page
* Small fixes and code cleaning
Doesn't it match `.plugin-bottom-bar`, `.plugin-version` and `.plugin-settings`, too?

Anyway, I think an explicit selector or a common class would be cleaner for sure.