FolderFilteringAction

[chibenwa]

CF https://github.com/linagora/tmail-backend/blob/master/docs/modules/ROOT/pages/tmail-backend/jmap-extensions/jmapFilters.adoc#folderfilteringaction-object

We can now plan support for folder filtering action.

We should first get a design for this tiny feature.

Proposal:

Add a button that opens folder picker onto the filter page.