Allow to re-trigger a build from the build page

[stsewd]

This is mainly for builds from a pull request (maybe would be good to add this to normal builds too?), sometimes the build fails for reasons that aren't related to the PR like https://readthedocs.org/projects/docs/builds/10259706/.

The status of the PR would be marked with the red X because of that. Travis and other services allow you to trigger a new build to refresh the status.

Currently, the only way to trigger a new build is by making another commit.
We could allow to re-trigger a build only on unsuccessful builds or maybe in all builds.

[stsewd]

The link gives 404 now because we delete all builds when the PR is merged/closed :D

[saadmk11]

I think we can show a Restart Build button on the build detail page for the External versions and it should be only available to the maintainers of the project.

[pllim]

Hello. I was looking for this feature as well.

FWIW, I think allowing trigger for both failing and successful builds is useful. This is because sometimes as a maintainer, I want to know if new upstream release on PyPI would break an already successful PR build.

Also would be nice to have a "wipe cache" option. Currently some builds are failing due to invalid cache for pip 20.0. A wipe is necessary before rebuild.

[ericholscher]

This is really useful for the PR builder. I think we need to figure out a good way to ship this 👍

[chaen]

+1 on the feature. We are hit by this problem when the build crashes (e.g. Command killed due to excessive memory consumption) and it is a pity to have to re-run 50 other tests that passed just because of that.