Pattern management: Add "transfer ownership" option to manage dropdown

[ryelle]

The dropdown to manage patterns should also include an item to "Transfer ownership".

Current dropdown:

This still needs a flow designed for how to transfer ownership. Does this toggle a new modal? Should it have some explanation? We probably can't show a dropdown/autocomplete of all users on wp.org, so the input to pick a new owner would be better as a text input like what's used on plugins:

I've put this in the "creation" milestone, but IMO we can launch without it if necessary.

[jasmussen]

I'm jumping in a bit blind here, so forgive me if some of these ideas don't apply. I imagine that "Delete pattern" opens a confirm dialog, correct? In that vein, "Transfer ownership" is similarly destructive, and a modal seems like it might be a good fit. Here's a quick doodle:

Let me know if that's useful.