Add Group model
Add Membership model
User can join/leave group
Add simple_form gem
Setup for simple_form

$ rails g simple_form:install --bootstrap
Add gems for development
Update Procfile for development
fixup! f0c921f3b3c10c9c5d311f00bb6f6fb1edad2f91
Add locale file path
Add group & membership features
Update simple_form gem
Merge branch 'group'