Improve installation and config UX

Problem/Motivation

There are a few parts of the installation and setup process that could be streamlined:

1. Composer Manager requires an additional step to avoid "class/ autoloader → not found"-type errors.
2. Brandfolder configuration form does not allow users to explicitly select no default collection.
3. When configuring an image field for use with Brandfolder, admins must do two things: change the widget to "Brandfolder Browser" and change the field "storage location" (corresponds to a file scheme) to "Brandfolder (read-only)." These should remain two separate items, but we should probably either auto-link them or throw some kind of warning if an admin enables one without the other.
4. If any image fields are using Brandfolder, the site needs to use the included Brandfolder image toolkit. We should either automatically enable it or throw an admin message instructing people to configure it.

Steps to reproduce

See above

Proposed resolution

Review each item and address as feasible.

Remaining tasks

Review each item and address as feasible.

User interface changes

TBD

API changes

None.

Data model changes

None.