Pipeline Trigger API and UI with user permissions

We have Build Triggers API that hasn't been upgraded since: https://docs.gitlab.com/ee/user/project/new_ci_build_permissions_model.html.

FE: @kushalpandya

The new CI permission model allows us to fetch sources/images from all dependent projects. Build triggers, because they are user-less, use the old method of "permissions", which only allows fetching from the directly related project.

Idea

1. Change Build Triggers to Pipeline Triggers,
2. Add /pipelines/trigger endpoint to fulfill the 1.,
3. Deprecate /builds/trigger,
4. Extend Pipeline Triggers to store User ID of user that did create a trigger,
5. Allow editing of user which permissions are used for running a pipeline,
6. Allow editing of the description (we add description to make triggers named, and to know which trigger can be removed).

What is needed

1. A new /pipelines/trigger endpoint (For 8.17),
2. A new column for ci_triggers: user_id (For 8.17),
3. A new column for ci_triggers: description (For 8.17),
4. An ability to edit existing triggers (For 8.17),
5. With 9.0 remove code that is responsible for supporting deprecated user-less method of CI authentication.

Edge cases

1. If user will be removed we will nullify the ci_triggers.user_id making it to behave as previously or with 9.0 to generate a permission denied,
2. If user will get blocked or removed from group it will automatically loose permissions to run CI pipelines with this trigger.

Implementation

Design

cc @markpundsack @dimitrieh @grzesiek