Add text-message sending panel

There is now a module which allows the texting of broad subsets of
students using Twilio.
Clean up unnecessary files
Make changed suggested by review

Un-hardcode email address, throw descriptive error if not properly
configured, remove dead commented code.
Merge remote-tracking branch 'hobinjk/group-texting-module' into group-texting-module
Add twilio to requirements.

Note that the Twilio API Python client is free software.
Some minor adjustments to the text messaging module.

The user selection now uses the regular user search controller instead of selecting specific lists. Instead of printing to console, what was printed will instead be displayed on the finalization page. The user interface is now three-stage instead of two-stage, with an intermediate stage where the user can double-check the number of users who will be sent a text message.
Group texting module: remove template files that are no longer used.
Fix send_text_messages script.
Merge branch 'main' into group-texting-module
Merge branch 'main' into group-texting-module
Add configuration check to group texting module

The module uses Twilio and requires 3 settings:
- TWILIO_ACCOUNT_SID
- TWILIO_AUTH_TOKEN
- TWILIO_ACCOUNT_NUMBERS

If these aren't set properly, the module won't work, and I think it is
good to explain this in case it happens.
Display estimated time before sending text messages

In previous versions of the code, the time estimate was sent after all
messages had been sent, which was not very helpful.
Add placeholder Twilio settings in django_settings.py
Improves filtering of Records

Adds a Record.filter() classmethod as a wrapper around
Record.objects.filter(). In addition to filtering by all the fields of
the Record model, it also accepts an optional bool parameter only_today,
which if True filters for Records only from the given day, which can be
useful for applications such as teacher check-in.

Modifies Record.user_completed() to also accept the only_today
parameter, and to use Record.filter() internally rather than construct
its own QuerySet.
Modifies teacher check-in to grant Records by day

Uses the new/improved Record.filter() and Record.user_completed()
methods to check for existing Records only for the given day.

Partially resolves Github issue #818.
Fixes multi-day teacher check-in

Only checkins from the date of 'when' (which defaults to the current
date) are considered, so that check-in can be reused each day/week of
the program. To see who failed to check in on a certain day / by a
certain time, admins can use the pre-existing "previous day" feature.

Resolves Github issue #818.
Sorts teacher check-in by time of class
Adds RecordTest class
Adds TeacherCheckinModuleTest

Starts by writing runCheckInTest(), which tests checkIn() and
undoCheckIn().

More tests forthcoming.
Merge pull request #1209 from learning-unlimited/issue-818

Issue 818