This PR adds the 'choose' permission discussed in #4488. It limits Image and Document choosers to showing only those Collections (and their contents) for which the current user has the Choose permission.
It grants Choose permissions to the Root Collection to every existing Group that has the "Can access Wagtail admin" perm, via data migration. So there will be no change in behavior unless the user goes in and alters the Choose perms for their site's groups.
This PR has no effect on anything except choosers. The Image and Document index pages are still controlled solely by the 'add' and 'change' permissions, as those are only accessible by users who have those permissions in the first place. This means that it's now possible to configure a Group to allow its members to choose images from existing Collections when editing pages, *without* them being able to edit said images or add new ones.
