Change the way Push folders are configured

[cketti]

Instead of configuring folders to belong to a certain "folder push class", we want to switch to a simple toggle for Push in folder settings. This in turn will allow us to get rid of the "Push folders" account setting.

(Probably incomplete) list of things to change:

• [ ] Folder settings
• [ ] Account settings
• [ ] PushController
• [ ] FolderRepository
• [ ] Settings import/export
• [ ] Database migration

See also #2669