Trigger build on default branch when saving a project

[dojutsu-user]

Close #5358

[dojutsu-user]

@stsewd
Tests are failing because every project don't have a default_branch.
I am thinking to add try.. except block.. if it fails then go for LATEST version.
Need your suggestions.

[stsewd]

I think it's the opposite, projects have an invalid default_branch

https://github.com/stsewd/readthedocs.org/blob/4fd56f6fe7c9a646a2f628a17fda07a56fcc49be/readthedocs/projects/models.py#L971-L973

Or something else, that we can fix in the tests.

[dojutsu-user]

get_default_branch() is returning master for git based projects which is not present as version for many projects.

[stsewd]

Ok, looks like the method is used for another purpose (to get the version to which point to latest, not latest itself). We need something like projec.default_branch or LATEST I think

[dojutsu-user]

@stsewd
I have implemented that and also added the test.

[stsewd]

Thanks

[humitos]

Looks good!

Please, update the docstring of the function prepare_build:

    :param version: version of the project to be built. Default: ``latest``

since it's not latest the default now.

[dojutsu-user]

@humitos
Updated the doc string.