Keep track of all groups, not just "important" ones.

We'll need this if we want to allow maintainers to allow only specific groups
to manage their Stacks.
stacks: Move some code from save_stack into Stack.update_packages
models: Create a new BodhiBase.update_relationship method.

This is used to add or remove items from a many-to-many relationship.
models: Add a Stack.groups relationship
stacks: Update the stack's packages and groups with our new method
stacks: Send fedmsgs when saving or deleting stacks
Merge branch 'feature/stacks' of github.com:fedora-infra/bodhi into feature/stacks
request: Return None from get_user in certain cases
templates: Tighten up the new stack form