Rate limit pipeline creation

Problem

It may become necessary for an Admin of a GitLab instance to limit how many pipelines can be triggered by a user due to resource constraints to keep the system stable. Currently that requires changing a hard coded limit in the source code.

Proposal

Create an admin interface to make this value configurable.

Set sane default limits for GitLab.com that balance user needs and overall stability of the system.

Placeholder issue for !86466 (comment 946103680)