Add RSpec tests for models and controllers

Adding model and controller specs.
You should probably rewrite this with `context` so it would be easier to follow. Example:

``` rb
context "when the user has a standup for today" do
  before  { user.standups.create }
  subject { user.standups.build }

  it { is_expected.to_not be_valid }
end
```

and similarly for other cases