Mark rollups with the `rollup` label on creation

[Centril]

See e.g. https://github.com/rust-lang/rust/issues?q=label%3Arollup+is%3Aclosed

We'll need to do it manually on beta rollups tho.

[Centril]

Pointers:

• Pulls are made here:

  homu/homu/server.py

  Lines 334 to 339 in b89e01b

  	pull = base_repo.create_pull(
  	title,
  	state.base_ref,
  	user_repo.owner.login + ':' + branch_name,
  	body,
  	)

• Search for github.add_label and add a call to that in ^---.