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
Remove try/except block for twilio import

No reason not to import twilio at the top of the file.  twilio is already
in requirements.txt.
Merge pull request #1055 from learning-unlimited/group-texting-module

Group texting module