Remove the redefinition of `#name` on Submission

Submission includes the `Named` module already, so there is no need to
redefine the `name` method that's included from the `Named` module.
Use the `Named` module in UserExercise

`UserExercise` defined a `name` method that is exactly same as the one
in the `Named` module. Convert `UserExercise` to use the module instead.
Merge pull request #1724 from mattr-/remove-unneeded-name-methods

Consistently use the `Named` module to provide the `name` method