Add Load Performance Testing

What does this MR do?

Closes #10683 (closed)

Adds in new Load Performance Testing feature based on k6 to go alongside existing Browser Performance Testing.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Database Migration Outputs

Migration

== 20200707095849 AddLoadPerformanceToPlanLimits: migrating ===================
-- add_column(:plan_limits, "ci_max_artifact_size_load_performance", :integer, {:default=>0, :null=>false})
   -> 0.0023s
== 20200707095849 AddLoadPerformanceToPlanLimits: migrated (0.0024s) ==========

Rollback

== 20200707095849 AddLoadPerformanceToPlanLimits: reverting ===================
-- remove_column(:plan_limits, "ci_max_artifact_size_load_performance", :integer, {:default=>0, :null=>false})
   -> 0.0115s
== 20200707095849 AddLoadPerformanceToPlanLimits: reverted (0.0154s) ==========