Propagation edit menu: improve clarity

[smadbe]

Overview

In the propagation edit (alg-propagation-edit-menu):

• add a textual explanation below the options
• grey-out unavailable option instead of hiding them
• fix small ui glitch

Note that we will, in a next request, add a button (at the bottom?) for advanced configs.

Acceptance criteria

• Add the following explanation in the "menu" between the title and 3 options, in small, white?: "If you have the right, choose below how users will see this content within the chapter. Note that they will see it regardless if they have access to it in another chapter or have obtained rights that unlock it." (in French: "Si vous en avez le droit, choissez ci-dessous comment les utilisateurs voient ce contenu au sein du chapitre. Notez qu'ils le verront quoi qu'il arrive s'ils y ont accès dans un autre chapitre, ou ont obtenu des droits qui le débloque.")
• when some options are not available to the user, grey them out (make it clear it is non-clickable) instead of hiding them
• (if not too long) fix the glitch when the "menu" displays on the top of the button: