Create More Auth Utility functions to convert between email and UID

[keiffer01]

Issue summary

Utility functions need to be made that allow any user's UID to be retrieved given their email and vice versa. This is to be used when storing collaborators for a trip, so that users can input emails for collaborators and developers can convert these to UIDs to store instead. Then when retrieving a trip, the UIDs can be converted back to emails. The purpose of this is to prevent a change of email from locking a user out of their trips. This will likely need to use the Firebase Admin SDK and possibly be implemented in a Java servlet.

The specific names of the functions are:

• getUserEmailArrFromUserUidArr(uid)
• getUserUidArrFromUserEmailArr(email)