Notifications: email about DONT_SHALLOW_CLONE deprecated feature flag

[humitos]

This PR prepare an email notification to be sent to users with projects using this feature flag. We use a similar pattern than the one used for the deprecation of the configuration file: 1 email per user listing all their projects affected ¹.

There are set of different changes in this PR (each set has it's own commit to make it easier to review):

• Expand the contact_owners.py management command
  • Follow the convention we have in other notifications regarding the context: us production_uri instead of domain
  • Add all the projects for a particular user to the context, so we can show actions specific to those projects
  • Use the internal get_context_data() and extra_context= to follow the pattern stablished for the Notification class.
• Add a template filter to check if a Project has a feature flag or not
• Write an initial email to be reviewed by the team

Preview

Related: #9779

Footnotes

1. I'd like to standardize this pattern in a generic way somehow, but I don't want to deviate too much right, so I made some small changes to allow me to do this in a simple way for now. ↩

[stsewd]

I think we should stop trying to adapt the command each time we want to send an email, and just call it from a script, as mentioned in #9882 (comment).

[humitos]

Is expanding the script a problem? This PR does not make breaking changes on the script; it just add functionality. This also helps to keep the script maintained and working when we need it.

[stsewd]

This is injecting all projects by default in the context, and adding a filter, which shouldn't be required for all types of emails/notifications. This can be just a custom context function, with all the proper projects already filtered.

[ericholscher]

I tend to agree... it seems like if we're not going to be sending these emails in an automated fashion in the application, just having a script that we run to send them is a lot simpler, and doesn't require a deploy or code changes. It would make it much easier to iteratively send notifications to each of our feature flag users in a pretty quick way, and we could keep the scripts in VCS if we wanted to review/version control them. I don't feel super strongly, though, but I do see how requiring deploys makes it more annoying to ship these notifications.

[humitos]

OK. I'm closing this PR and I will create a different script. I opened #10470 with the changes required to follow the conventions we have and keep consistency.