Automatically block and close PRs from develop -> stable

[hwikle-lanl]

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

[hwikle-lanl]

Actually, we will occasionally need to merge develop back into stable.

[hwikle-lanl]

Reopening because I think we might want this after all.