Feature/calendar ui integration

Hi

This is merge of all earlier calendar pull request and includes Calendar integration with UI button to authorize calendar.

The deployments need migration (need to comment out src/requirement.txt entries, rerun vagrant, and put them back to deploy to AppEngine).

Also Calendar API needs to be enabled in AppEngine in section "API" in API & Auth
Client ID/Client Secret in "Client ID for Web Application" section "Credentials" in API & Auth
Redirect URIs is set to <URL root>/oauth2callback/calendar using Edit Settings
Javascript Origins is set to <URL root>
Key for browser and server applications is also generated (may be optional)

Regards
Mouli