* Add unit test coverage for this "exclusion from tracking" logic
* Exclude based on regex match instead of raw string comparison
* Exclude heartbeat requests from tracking

Also added datadog application to commom django apps and set it to use
a local statsd server by default.

A one-click way of getting to editing the course

Includes Data + Schema migrations for optout email -> user.id change.
Note that migrations should be reversible.

Adds a new Email link to the instructor dashboard for frontend interface to send
email to course members. Adds a feature flag ENABLE_INSTRUCTOR_EMAIL to toggle this.
Creates a new djangoapp bulk_email that handles this action by getting the recipient
list and batching the emails to different celery tasks to do the actual sending.
Requires lynx package to convert HTML email to plaintext. Handles SMTP errors by
retrying or falling through to the next email. Adds the option to opt out of course
specific emails in the user dashboard with an Email Settings link for each course.
Uses severable configurable settings with defaults. DEFAULT_BULK_FROM_EMAIL
specifies the from address for email. EMAILS_PER_TASK specifies the number of emails
each celery task takes on. EMAIL_HOST, EMAIL_PORT, EMAIL_HOST_USER,
EMAIL_HOST_PASSWORD, and EMAIL_USE_TLS for the SMTP email backend settings.

Co-authored-by: Akshay Jagadeesh <akjags@gmail.com>

not common.  This ensures that we load the MITX_FEATURES
token BEFORE deciding to disable CSRF.

to cms settings (it was throwing 500s for Stanford when the CMS
calls render_from_lms)

especially since s3 namespace is _global_

wsgi_apache.py passes an apache env variable for SERVICE_VARIANT
to the os environment, where it's normally set when we use gunicorn

* Moving to the auth tokens section
* Removing the feature flag logic since all features are loaded earlier
```
    for feature, value in ENV_TOKENS.get('MITX_FEATURES', {}).items():
        MITX_FEATURES[feature] = value
```

Tell segment IO whenever a page is loaded.  Once this works, we
can add more detailed instrumentation for events we care about.

Will be turned off in prod until the SEGMENT_IO_LMS feature flag is
set to True and a SEGMENT_IO_LMS_KEY is set.

Rather than require the `PLATFORM_NAME` to be specified in the
`ENV_TOKENS`, allow the default setting in `common.py` to be used.

Themes do not necessarily want all of the available LMS routes, such
as `/jobs` and `/university_profiles`. This change splits up the
`lms/urls.py` file and selectively enables/disables routes based on
whether or not a theme is enabled. This is a naive solution for now;
a better solution gives themes a way to selectively overrides such
routes.

Additionally, with the `MKTG_URL_LINK_MAP` setting that hits certain
routes immediately on each page render (whenever the `marketing_link`
helper function is called), themes may crash if they don't leave
all marketing link routes present in `lms/urls.py`. This change also
provides the ability to override the `MKTG_URL_LINK_MAP` in the
settings.

Finally, modify the mitxmako marketing URL middleware to not try to
reverse disabled URLs, which are those keys in the map whose values
are `None`.

Allow themes to override contact and bugs email addresses via the
settings.

Configure the technical support email address in the settings so that
themes can override with an email of their own in the appropriate
env.json file in production.