Rework around skipping a build

[humitos]

Currently, we have something like if project.skip: return None in prepare_build which is then handled at trigger_build helper. It's not a really good workflow and we probably want to raise an exception instead and manage it in a proper way.

https://github.com/rtfd/readthedocs.org/blob/14942e9c1322a320ec3ecd34fe6a797e975d3a6f/readthedocs/core/utils/__init__.py#L98-L104

Additionally, we were thinking on managing this from a signal or similar that could be extended from outside. As an example, in .com we will probably be using something like organization.disabled where we will want to skip those builds also.

Also, the error message shown in the project detail page should be extended to show different messages depending on the skip reason. Currently, it's fixed to resource usage.

Ref: #4255

[stsewd]

Related #4991