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
I would move that to the previous line.

    textarea, input:not(.i-button):not([type="checkbox"]) {

looks more readable to me. Also, what about radio buttons? Probably they need the same handling checkboxes get?