Add maintainer_can_modify field to PR creation

[dsheets]

There is now a maintainer_can_modify field in a PR creation request. This is necessary to complete ocaml-opam/opam-publish#44.

[yallop]

It'll be useful to have this, but maintainer modifications appear to be on by default, which makes it slightly less urgent.

[dsheets]

Interesting... that appears to also have changed. Is there more work needed to close ocaml-opam/opam-publish#44, then?

[yallop]

There's no more work needed, so I've closed the issue.