Add indexing for the mimetype term

Add indexing for the mimetype term

This adds the indexing support for the "mimetype:" term and removes
the broken test flag.  The indexing is probablistic in Xapian terms,
which gives a better experience to end users.  Standard content-types
of the form "foo/bar" are automatically interpreted as phrases in
Xapian due to the embedded slash.

Assume, separate messages with application/pdf and application/x-pdf
are indexed, then:

- mimetype:application/x-pdf will find only the application/x-pdf
- mimetype:application/pdf will find only the application/pdf
- mimetype:pdf will find both of the messages