"Mismatched labels and translations" export failure when select_*_from_file present in multi-langauge forms

[jnm]

It looks like what happened is that MEDIA_TYPES was originally added with the right stuff:

MEDIA_TYPES = ('audio', 'image', 'video')

And then, in the same PR, those were prefixed with media:: and added to the list of translated columns 🆗
https://github.com/kobotoolbox/formpack/pull/280/files#diff-5382e2c7532fd5627ec49791c7f53cc8c5a39f865903c702bf1151be4dccfd99R177-R194

However, yours truly recently saw that we had two constants both named MEDIA_TYPES and thought that one was just an incomplete version of the other. So, I metí la pata and combined them in #321, which is the real source of the current problem.

We clearly need two constants: one for all question types that collect media files (that's what MEDIA_TYPES is currently, after #321), and another constant for media COLUMN names.

We also need, unfortunately, special handling for existing asset versions that have media::file in the translated list 😐 in order to work around the existing damage.

To reproduce the problem, create an XLSForm with this in the survey sheet:

type	name	label::Ukrainian	label::English
text	interviewer_name_text	Введіть ім'я співробітника:	Enter interviewer's name
select_one_from_file oblast.csv	oblast	Область	Oblast

Create oblast.csv with the following contents:

name	label::Ukrainian	label::English
ck	Черкаська	Cherkasy
zht	Житомирська	Zhytomyr

Import the XLSForm into KoboToolbox and add oblast.csv as a form media file. Deploy, and create a submission. Attempt to export, see the following failure:

formpack.errors.TranslationError: Mismatched labels and translations: [Введіть ім'я співробітника:, Enter interviewer's name] [Ukrainian, English, None] 2!=3

Internal discussion: https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/Critical.20export.20failure