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

* Cleaned issues and issues_statistics docs
* Renamed param with_labels_data to with_labels_details
* Added spec for N+1 check when retrieving labels from issue
* Refactoed CheckAssigneesCount validation class

Remove label_name and milestone_title params support
Add mutually_exclusive validation for author_id and author_username
Add mutually_exclusive validation for assignee_id and assignee_username
Add validation to allow single value for asignee_username on CE
Add separate issue_stats_params helper for statistics params and
reuse in issues_params.

Cleanup IssueBasic entity to keep it basic and move extra attributes
to Issue entity which contains more details

When issues_controller endpoint was used for search, the parameters
passed to the controller were slightly different then the ones
passed to API. Because the searchbar UI is reused in different
places and builds the parameters passed to request in same way
we need to account for old parameter names.


Add issues_statistics api endpoints

Adds issue_statistics api endpoints for issue lists and returns
counts of issues for all, closed and opened states.


Expose more label attributes based on a param

When requesting issues list through API expose more attributes
for labels, like color, description if with_labels_data param is
being passed, avoiding this way to change response schema for users
that already use API.

https://gitlab.com/gitlab-org/gitlab-ce/issues/57402