Send proper context to celery email notification task

[humitos]

This solves an issue in the .com site that we are experimenting.

Although I couldn't get the error report in .org Sentry, I'm not receiving the email on FAILED builds in my .org account either. So, I think this PR will also fix the problem here.

NOTE: by sending the objects as dictionaries we can continue using the templates as they are. Antoher approach could be sending all that data in separate fields like version_verbose_name or project_name and adapting the templates to this.

Ref: https://github.com/readthedocs/readthedocs-corporate-ops/issues/77

[agjohnson]

I think this looks good! The context data structure makes sense to me.

I thought we solved this problem here already, but perhaps this was a different email i'm thinking of.