Vote for articles and comments

As a User I'd like to be able to vote for articles and comments so that I can both express my opinion on the quality of the article/comment and see the opinion of others.
Acceptance Criteria:

• Upvote and Downvote buttons are available, they should be clearly visible since it's going to be one of main functions, user should want to press them
• User can change his vote if he voted yet, but can't just remove his voice
• The sum of votes is shown on some visible place, it can be positive or negative depending on the sum of upvotes and downvotes

No changes are required for the page where articles are listed, we're going to do this in scope of other issues.