do not load Bootstrap in non-admin controller for non-bundle themes.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #2041
| License       | MIT
| Doc PR        | -

Aren't you normally the guy who says: "Try to avoid every single BC-break as hard as possible!"?