Restrict the field formatter to media entity type, or make it work for other types

[donquixote]

Currently the CoolPreview field formatter does not enforce the entity type on which it can be used.
However, if the entity type is something other than media, it won't work correctly.

Two options:

1. Implement isApplicable() method to enforce the entity type.
2. Change the formatter so that it works for other entity types.