Add test support for the send-reminders script.

[NathanWEdwards]

This pull request contains changes to offer Jest test support to the Courtbot VT project.

Development dependencies have been added to package.json for test and versioning of existing packages have been updated. The following dependencies have been added:

• jest, ts-jest and type declarations as a test framework
• async-mutex for handling concurrency

The send-reminders.ts script containing an immediately invoked function expression has been modified for test support enablement. This request modifies send-reminders.ts to expose the content that is present in the grouping operator of the IIFE, as an exported function of the module, when under test.

Several tests have been added and initial mock representations have been added for Mongoose and Twilio.

[iamyume]

finally taking a look at this. So far it looks good but it might useful to add a test command in the scripts section of package.json