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.
Add balance breakdown to /accounting/ for each account.

See Issue #874.
Alters lunch constraints so they can happen on only certain days.
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 issue874
Make accounting summary view @admin_required
Add Formstack client library and some models/methods to interact with Formstack forms
Add a FormstackAppSettings module_ext to store metadata and settings for a Formstack application form
Add FsStudentApp object to store data related to a student's submission
Add FormstackAppModule and /studentapp handler
Improve error handling in Formstack client library
Trivial: add a docstring and some whitespace
Cache field info in FormstackAppModule
Move get_field_info and get_student_apps from FormstackAppModule to FormstackAppSettings for easier access