During a reshuffle be able to explain to users that the contents of this page may change/be temporarily innacurate

This is feature complete UI, with placeholders in the controller to demonstrate what the Admin will need to provide:

* Boolean flag to control if the feature is enabled
* Text of Govspeak/Markdown that can be set within the admin, to be displayed
** Copy and links in this  will vary depending on cicumstances
Hide the live minister counts during a reshuffle

The numbers will be updating as changes are made and it'll be less confusing to hide it during this time. Maintain the link to the ministers page, which will have the more specific reshuffle messaging
Make .odp an allowed and indexed file type

We already allowed editors to upload OpenDocument documents (odt) and spreadsheets (ods). Presentations (odp) should also be allowed to be uploaded and indexed.
Add .odp file type description
Backend work for minister reshuffling
Additional test for sitewide settings update method
Merge pull request #1492 from rooreynolds/patch-1

Make .odp an allowed and indexed file type
Merge pull request #1491 from alphagov/reshuffle-messaging

Minister reshuffle messaging

https://www.pivotaltracker.com/story/show/70928098
Fix attachment upload test
Merge pull request #1495 from alphagov/fix-attachment-uploader-test

Fix attachment upload test