refs https://github.com/TryGhost/Team/issues/1107
- updated schema validation to allow `'image'` through as a known setting type now that Admin has support
- added transformation of setting values for `'image'` types because they will be URLs and should be stored with `__GHOST_URL__`
refs https://github.com/TryGhost/Team/issues/1106
- updated schema validation to add `'boolean'` as an allowed `type` value
- added `format()` and `parse()` methods to `CustomThemeSetting` model to match `Settings` model behaviour for boolean-type settings
- this is a small part of a bit of cleanup of our test files
- the goal is to make the existing tests clearer with a view to making it easier to write more tests
- this makes the test structure follow the codebase structure more closely
- eventually we will colocate the tests as we break the codebase down further
