Store email contents in catalog SearchableText

Currently the raw email in the Email content type is stored as SearchableText in the catalog. The raw email contains a lot of stuff we don't want indexed. 

I've changed the Email content type, an adapter is called to return a string which is used as SearchableText by the catalog. The adapter used the email browser view to the email body and headers, both are returned as searchable text. I've also added tests for my changes.