Notifications: default behavior for PR versions

[humitos]

We've talked about what would be a good default behavior for notifications on pull requests (external versions):

• I (humitos) think the current behavior is fine (enabled by default) and it doesn't bother me too much
• Eric mentioned he wants it to be disabled by default

I'm not super strict here tho. In particular, because we already have some other UI elements (file tree diff) that also "communicates" in a not-too-clear-way tho, that this is not the production version.

If we want to move forward with Eric's proposal we need to define how to change the current default state, since all the projects have it already enabled. Would we change this behavior only for new projects?

• Related: Notification: render it inline using docTool.getRootSelector() #552
• Model where we make it enabled by default: https://github.com/readthedocs/readthedocs.org/blob/29a912f4aad30b83b29539cf4ab46c5f7721f040/readthedocs/projects/models.py#L262