Multi-user support

[maxbbraun]

• User account management for addresses and other settings
• Calendar API auth flow
• Authentication between client and server

[maxbbraun]

First steps added with 882c0fe51839950e4e98dd1681a8d29021985816

[maxbbraun]

Some remaining work:

• Randomly generate and expire/refresh device keys (store in NVS)
• Tie clients to user accounts, e.g. Google accounts with device codes