Adds a link to GUI standards to the PR template, at a later date we can
remove this as technically it falls under coding standards. But to
spread the word initially it makes sense to explicitly draw attention to
it.

Was in the wiki as
https://www.mantidproject.org/Individual_Ticket_Testing

The PR template should probably point to the actual coding standards.

Ensure the hidden text is actually hidden

Clarify what to do about the release notes

Link to release notes directory in the source tree.