New `Api.projectMergeRequests` allows:
 - to query for all MRs on a project without specifying `mergeRequestId`
 - to filter the returned MRs using parameters from
 https://bit.ly/2H7We4V

 The new API request is used for fetching information about MRs
 associated with a particular branch in Web IDE to have IdeSidebar
 behave consistently in both scenarios:
 - getting to a branch from and MR (where we already have info about
 relevant MR), or
 - getting to a branch somehow differently directly

For cases where there are several merge requests that the current branch
is associated with, mark the most recent one as 'current'

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49663