Submitted supporting pages are active as well

This fixes a bug where submitted editions were being excluded from the
list of active supporting pages listed in the admin interface.
Fix the cause of a couple CSV preview exceptions

1) Because only the first 10 lines where sampled to guess the
encoding, it was sometimes getting it wrong (i.e. when windows chars
are not present until later in the file). Now the entirety of the
preview is used to check the encoding, increasing its accuracy.

2) Some CSV files use a mixture of \r\n and \n (lolz), which confuses
the CSV library and causes it to throw an exception when the offending
row is reached. Instead of waiting for this to happen when the preview
is rendered in the template, CsvPreview now raises the exception at
the point it is initialized, allowing it to be handled more
gracefully.
Data migration to restore alt format providers

Previously, the alternative format provider was set on the policy and
infered from there. Now, it must be explicitly set on the supporting
page. This data migration sets the alternative format provider on
those supporting pages that don't have one, based on the value of the
parent policy.
Remove redundant code

Policies have no truck with alternative format providers anymore.
Merge pull request #1095 from alphagov/61301412-include-submitted-supporting-pages

Submitted supporting pages are active as well

[Fixes https://www.pivotaltracker.com/story/show/61301412]
Merge pull request #1096 from alphagov/61539604-csv-preview-fixes

Fix the cause of a couple CSV preview exceptions

[Fixes https://www.pivotaltracker.com/story/show/61539604]
Merge pull request #1097 from alphagov/61219962-restore-afp-for-supporting-pages

61219962 restore afp for supporting pages

[Fixes https://www.pivotaltracker.com/story/show/61219962]