Multiple Instances in MSAL Pt. 1 - Cache Refactor and Adding Unique Keys

[pkanher617]

This PR is the first in a series to support multiple instances of MSAL in the same window stack (i.e. in main window and iframe, two separate iframes, etc.).

This PR covers a few changes:

• Cleaning up the Storage class to make room for future improvements
• Introducing new functionality to make unique keys in the cache tied to each instance of MSAL
• Cleaning up the Constants.ts file to properly organize string constants and cache keys
• Introduction of new Storage mechanics to UserAgentApplication
• Migration from old cache signatures to new ones
• Unit tests

[sameerag]

Feedback:

• Better naming for AuthStorage.ts and MSALStorage.ts
• Retain writing to old cache values to facilitate rollbacks

[DarylThayil]

all in all, code looks good to me and we talked through design yesterday. If others are happy with where we landed so am I