Allow owners to do a "ready to merge" self-merge

[elibarzilay]

Add text to the comment telling owners that it can be done --- except
when there are too many owners, or when there are multiple packages.
(Owners can still do it if there are many, only multiple packages block
the whole thing.)

Closes #98.

Also remove an unused const approvals line.

[orta]

Cool, yeah - the code looks right to me, but I'm a bit unsure about why we get a lot more new comments in the tests when the fixtures didn't get updated

[elibarzilay]

There's a bunch of ignorable changes since there is one un-conditional new \n in the string now. Could be removed with

and I'll merge this PR almost instantly. Thanks for helping out! :heart:${
otherOwners.length === 0 ? "" : `\n
(${otherOwners.map(o => "@" + o).join(", ")}: you can do this too.)`}`});

But I suspected that it's too much. (I'm fine doing this change it if you think it's worth it.)

[orta]

👍 - yeah, probably that the SHA changed in the body and so it didn't get detected by the duplication lookup