Migrate client-side auth to msal.js v2

[scale-tone]

In other words, from implicit flow to auth code flow with PKCE.

Pull request for that is ready: #119
But sadly, this change would require reconfiguring existing AAD app registrations, until which a DfMon instance will get broken, once the new code gets deployed to it. Therefore let's postpone this migration.

To clarify: DfMon's client side supports both server-directed login flow and client-side auth (done with msal.js). This whole matter applies only to the latter case. While server-directed flow is sort of recommended, is used by default and won't get anyhow affected by whatever changes we make on the client side.

[scale-tone]

Closing in favor of microsoft/DurableFunctionsMonitor#4