warehouse: Allow maintainers to craft project tokens

[woodruffw]

Closes #6262.

cc @di / @dstufft / @brainwane

[brainwane]

The token management screen links to manage/project/[project name]/releases/ (the link provided in the "scope" item for a project-scoped token) but, as a maintainer, that gives me an Access Denied: "You don't have permission to view this page" error. Perhaps it should link to project/[project name]/ if the user is a maintainer. Or if that's inelegant, it should link to that page for all users.

[woodruffw]

Perhaps it should link to project/[project name]/ if the user is a maintainer. Or if that's inelegant, it should link to that page for all users.

Done: I've linked to /project/{name} unconditionally, since retrieving the user's role for each project would be a little inelegant in this context (and I think consistency is best, since we're filling out a single row within a table).

[brainwane]

Tested on localhost and I can successfully -- as a project maintainer -- create a token and upload a distribution with it.

[woodruffw]

@di should be good for another pass!