Trigger build on default branch when saving a project

[stsewd]

Currently when we save a project form (admin panel),
it triggers a build to latest

https://github.com/rtfd/readthedocs.org/blob/9874b866fea9696fa8495d7b3699f1bf1a3f923d/readthedocs/projects/forms.py#L69-L74

https://github.com/rtfd/readthedocs.org/blob/9874b866fea9696fa8495d7b3699f1bf1a3f923d/readthedocs/core/utils/__init__.py#L97-L98

Even if latest is deactivated, we should trigger a build to the default branch instead.

[dojutsu-user]

@stsewd
I'm working on it.

[dojutsu-user]

@stsewd
Does the build needs to trigger build for both the default_branch and latest branch ?

[humitos]

@dojutsu-user I think it should trigger just the default_branch without latest.