Adding utility method to Unit class

This is in response to a third party developer who is
pulling data from our Fedora Repository.  Not knowing whether a
given MasterFile object (in Fedora) has a transcription
datastream, he was getting timeouts and 404 errors among
his requests.

This method allows me to quickly add whitespace boilerplate
transcriptions to our objects within a unit that lack such data,
and will update the corresponding objects if they exist in the repo.

Called from the console at the moment, but could easily be
added to an admin page.
Making our logs more truthful.

Adding the unit.archive.name seems like a no-brainer.  The
extra caution in all these nil guards is just to keep this
processor from breaking if we make any unusual changes to
the Archive class.
Rearranged to use in one more place.
adding infomration methods
Allow creation of blank, non-null datastreams.

Corks and Curls project raised the problem of heterogeneous
object collections, where it could not be assumed that all
MasterFiles would return a non-error response to a call
for their transcriptions.  Going forward, we're adding
some whitespace and ensuring transcription datastreams are
built, unless the database field is empty.

TL;DR: if you don't want to add transcriptions to Fedora,
set :transcription_text => nil
watch your logic!
noticed log messages were not reporting unit id
Migration: Units now habtm LegacyIdentifiers

Need this for the Reed Yellow Fever collection, but I expect it
will be useful elsewhere in future.
Updating submodule.

jbo3t working on FBJ collection discovered we were adding
an unnecessary display term.  Delete from Holsinger XSLT