broadcast: add field for slate video upload

[Saxon Milton]

We will need to be able to upload a custom slate to be sent to vidforward. This change will add this field to the UI, and on the back end, utilise a request offered by vidforward (TBD) for slate update.

[Saxon Milton]

Merged in slate-request (pull request #234)

Add slate upload feature to broadcast page

Resolves issue #204

Approved-by: David Sutton

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