Differentiate enclosures based on content type.

Differentiate enclosures based on content type.

Some RSS feeds contain multiple enclosures with the same URL. When the first of
these is not recognized as an image, later entries are not added to the
database as rows in ttrss_enclosures. This change differentiates enclosures
based on their content type, so an entry can have multiple enclosure types with
the same URL (but possibly a different content type).