Add project level summary api

Closes #1076 (closed)

Add project level summary api for performance analytistic

Summary api path *namespace_id/*project_id/-/analytics/performance_analytics/summary.json

params:

• branch_name
• start_date
• end_date

return data:

{
  "commits_pushed": 0,
  "commits_pushed_per_capita": 0,
  "issues_closed": 0, 
  "merge_requests_merged": 0
}