Add new transfer metadata DB tables

These new tables support the transfer metadata form feature.
They allow taxonomies, containing arbitrary sets of terms, to be
defined and then associated with a given transfer.

One taxonomy (disk image metadata) is defined by default. Later commits
will introduce a form that allows disk image metadata to be assigned
to a transfer and edited.

refs #5900
Add term administration pages

These pages allow the existing taxonomy and taxonomy terms to be
browsed and edited.

Note that, while these exist in the source and are enabled by going
directly to their URLs, they are not actually linked to from anywhere.
The feature is considered too specific to be generally useful right
now.

Consider this a secret feature! If you're a source-diver, you can find
the URLs and try it out.

refs #5900
Write transfer metadata to disk and METS

This adds two new kinds of metadata references to the METS.

* Transfer metadata (created via the disk image metadata form) is
  directly embedded in the METS via mdWrap elements.
* Arbitrary XML metadata files can be included in a transfer's
  metadata/sourceMD directory. All files present there will be referenced
  via <mdRef> elements, but will *not* be directly wrapped in the
  METS.

Both kinds are referenced from the "objects" directory of the AIP in
an <amdSec> element.

refs #5900
Add transfer metadata edit form

This form allows transfer metadata to be associated with each
component of a transfer.

refs #5900
Clean up unused metadata transfer forms

These may be orphaned if a user started editing a form, but did not
save it.

refs #5900