Add Accepted and Rejected tabs to MR lists.

Before, MRs were referred to as Merged or Closed, but the Merge button on the MR page read "Accept Merge Request", the activity feed even read "X accepted merge request Y", and the Closed tab on the MR index included both Merged and Closed MRs. I've changed every occurrence to consistently refer to MRs as either Accepted or Rejected, which is less technical and more clearly two opposites.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2201.