Changed govspeak URL in document edit interface

Changed govspeak URL to point to the relevant guide for Whitehall editors. It is currently pointing to the markdown for GDS Content Designers using Publisher.
Handle unhandled feed types

We want to reject feeds for formats that we do not currently support,
e.g. document collections.
Use `https` instead of `http`
Merge pull request #1518 from alphagov/handle-unhandled-feed-types

Handle unhandled feed types
Merge pull request #1517 from alphagov/govspeak-url

Changed govspeak URL in document edit interface
Don't assume document at slug exists

This prevents exceptions being raised when an attempt is made to preview
an HTML attachment on a document that does not exist.
Prevent preview of access-limited HTMLAttachments

This correctly scopes HTMLAttachment preview to the loaded edition and
also enforces permissions check for access-limited publications.
Don't raise when we can render
Handle pdf-reader gem errors

The pdf-reader gem will not handle every single PDF. Here we rescue the
two common errors caused by strange PDFs and carry on. There is little
point reporting these errors to our exception tracking as there is
little we can do about malformed or unsupported PDFs.
Merge pull request #1520 from alphagov/handle-html-attachment-preview-on-non-existent-docs

Handle html attachment preview on non existent docs
Merge pull request #1521 from alphagov/handle-pdf-reader-issues

Handle pdf-reader gem errors
Better scheduled publishing error reporting

Currently much of the context around errors raised during scheduled
publishing is lost because the error message of the exception raised is
not reported.
Merge pull request #1522 from alphagov/more-info-in-exception-notifications

Better scheduled publishing error reporting
Remove UKVI alternat top tasks from org model

No pivotal ticket for this