Create migrations and models for group and group membership
Add groups list

* Add groups controller
* Add groups#index action
* Create simple view for groups index
Users can create groups

* Add routes for groups: new, create
* Implement groups#new and groups#create actions
* Create view for new group form
* Add link from groups index page to new group form
Users are members of groups

* Update groups#create to add current user as a member when creating a new group
* Display a user's group memberships on the user show page
* Refactor nav header for clarity
Add validations for group fields
Refactor group#create to clarify adding group membership
Users can join any group

* Add route for group_memberships#create
* Create group_memberships controller
* Add create action to group_memberships controller
* Make group lists into links on groups index and users show pages
* Extract partial for displaying a link to a group's show page
* Create groups show page with link to join a group
Only show Join Group link if not a member
Display list of group members on group pages
Add uniqueness validation to user email
Refactor joining a group

* Use member route for group_memberships
* Modify join link in groups show page for clarity
* Abstract method for joining a group in group_memberships controller
* Create method in users model to add user to a group
Show flash messages in application layout
Add Dashboard

* Create dashboards controller
* Create dashboards show view
* Redirect to root route after sign in (in sessions_controller#create)
* Add root route to dashboard, but only  when signed in
* Update header link text in application layout
Images can be added to groups

* Create migration for ImagesGroups relation
* Update image and group models with HABTM relationship
* Update images form partial to show groups checkboxes
* Update strong params in images controller
* Display images on groups show page