Firebase Dynamic Links for Event Share

[tasomaniac]

Firebase App Invite and Dynamic LInks related tasks are handled in
#637 and #668

The shared event links can also be integrated with Dynamic Links.
When the link is clicked in a computer, just the webpage will be open.
In a mobile device, the app will be installed first if it is not installed, then the event will be open.

A starting work is already done by @friedger here: https://github.com/gdg-x/frisbee/tree/feature/637_invites