Remove business_proposition display from lists and edition views

We've not been using this consistently and don't internally distinguish content
this way any more. Removing the use of the flag will be a step towards making
some changes to better flag the proposition, audience or other categorisation
of the content.
Remove distinction between business/citizen in email alerts

This leaves a little bit of lingering debt as we still have separate email
groups. I've not addressed that here as I think we should talk a bit more
about how we want to handle the email alerts and then any changes will involve
co-ordinating changes to config as well as to this code.
Fix weird test bug
Remove PublicationMetadata class as it added complexity

This one's a bit weird. It's included in publisher but referenced in govuk_content_models.
It dates from when there was a deeper separation between publisher and panopticon and
made a little more sense back then, but now it's a lot of code to extract a few fields
from a model.

There's a separate question of what we actually want to do with the metadata tab as it
doesn't feel quite right, but that's out of scope here.
Merge branch 'remove-business-flag'