Change Votes controller spec to expect redirects

After vote actions, the votes controller should redirect back to the
idea record's 'show' view. Adjust the controller spec to match this
expectation and to make no assumptions of what will be rendered.
Votes controller redirects to Ideas

The Votes controller should redirect to the idea's 'show' action to
render the view, not handle the view rendering itself.
Add comment note about Voting authentication
Add a 'change_vote' method to Idea model

Add a method for adjusting an Idea's rating by twice the value of the
vote's new value. The previous order of trying to subtract the value
then add it back didn't make any changes to the rating.
Fix syntax in Votes controller spec

Spec is missing an 'end' keyword towards the end.
Change '_vote' partial to use buttons

Use buttons to access vote actions instead of links.
Fix Vote feature specs

Rewrite Vote feature specs.
Merge branch 'votes4' of https://github.com/robinsonj/P4IdeaX into votes4