calendar builder where you can select the groups

[sayanee]

thanks @burnflare for the feature request 👍

Example: http://www.initializr.com/

Challenges / thoughts:

• where to get the group list?
  1. from the upcoming event list
  2. from the past active group list
• how to deal with new groups?
  1. store the previously selected groups in browser's localstorage / iPhone's memory

Implementation:

• a list of group_id can be passed on as params to /cal: code reference