The new API method projectUsers gets all the users
added to a particular project.

New API endpoint for merge request count
Updates all open tabs at the same time with one call
Restructured API response

API response changed to 401 if no current_user

Added API + JS specs

Fix for Static Check

Updated Count on Open/Close, Assign/Unassign of MR's

Checking if MR Count is refreshed

Added # frozen_string_literal: true to spec

Added Changelog

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/58583

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

**Why?**
Previously we simply concatenated our paths, which led
to requesting `//api/v4/...` and obviously failed. The BE
supports a relative_url_root of `/`. It's a bug that the FE
does not.

 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