Act on a flow line where it stands: update a dependency, follow or stop, pull a change in, send it back #9591

From #9434 box 2.

Done looks like:

• [x] Each line in the flows view carries its one action, posting to an existing action (following, origins, forks) through the usual same-origin check and permission.
• [x] Update this dependency opens an issue for the crew, or a pull on autonomous repositories.
• [x] Tests: each action works for a writer, is refused for a reader and cross-origin.

Files: src/web/pages/flows.ts, src/flows/, test/.

Reviewed pull #9619 at its head: ticked Each line in the flows view carries its one action, posting to an existing action (following, origins, forks) through the usual same-origin check and permission.; Update this dependency opens an issue for the crew, or a pull on autonomous repositories.; Tests: each action works for a writer, is refused for a reader and cross-origin..