Command contact_owners: add support to filter by usernames

[humitos]

Add --usernames argument to be able to filter by usernames when sending contacting users.

This can be used as:

django-admin contact_owners --notification notification.md --sticky --usernames usernames.txt

humitos
santos
anthony

Read the Docs is going to force email verification to be able to login accounts in the following weeks.
We've noticed **your account doesn't have any email verified**.
Please, go to [your email settings](/accounts/email/) and verify your email now.

In this case, it will send a sticky notification with that content to these 3 users.

Example

Required by #9865
Fixes https://github.com/readthedocs/meta/issues/104

[stsewd]

Not really sure if it's worth trying to support every use case on this command, I've been using the contact_users function from the django shell instead, this allows you more control over the users and context of the messsage, for example:

"""
Script to email owners of organizations
that have domains already on Cloudflare.

We are asking more about their setup.

Email at https://pad.riseup.net.....
"""
from pprint import pprint

import requests
from django.contrib.auth.models import User

from readthedocs.core.utils.contact import contact_users
from readthedocs.projects.models import Domain

pending_domains = [
    "example.com"
]


def get_email():
    url = 'https://pad.riseup.net/p/..../export/txt'
    content = requests.get(url).text.split('\n')
    email_subject = content[0].strip()
    email_content = '\n'.join(content[1:]).strip()
    return email_subject, email_content


def get_users():
    return User.objects.filter(
        owner_organizations__projects__domains__domain__in=pending_domains,
    ).distinct()


def get_context(user):
    domains = Domain.objects.filter(
        project__organizations__owners=user,
        domain__in=pending_domains,
    ).distinct()
    return {
        'pending_domains': domains,
    }


def email_owners(dryrun=True):
    email_subject, email_content = get_email()
    response = contact_users(
        users=get_users(),
        email_subject=email_subject,
        email_content=email_content,
        from_email="Read the Docs <[email protected]>",
        context_function=get_context,
        dryrun=dryrun,
    )
    pprint(response)

[humitos]

Gotcha! 👍🏼 I'm fine merging the changes in this PR into the script since it's not changing its core behavior, but just adding another generic way to get the emails that have to be contacted (--usernames). Next time, if there are some extra logic required for my script, I will create another file.

[ericholscher]

I think adding this makes sense, given that it's a pretty common use case. Probably worth saving Santos' example somewhere, and linking to it in a comment from this code.

[humitos]

@ericholscher Done! Added a new commit that extends the class docstring pointing to the demo script.