Add dependecy config option to bridge jobs

Adds a "dependency" keyword to bridge job configs that denotes an
upstream dependency.

Expose bridge dependency config in model

Add SeedBridge pipeline chain step

Adds a SeedBridge Pipeline chain step that seeds bridge jobs with their
upstream dependencies.

Rename bridged_job to bridge in service

Handle bridges with nonexistent projects

Handles upstream bridges that point towards projects that do not exist.

Check user permissions before seeding bridge

Checks if the user has permissions to read pipelines on the upstream
project before seeding bridge with upstream pipeline.

Rename SeedBridge to Subscribe

Renames the Pipeline chain step from SeedBridge to Subscribe to better
reflect the intent.

Use intent revealing interfaces

Extract subscribe implementation to service

Extracts all of the bridge subscription functionality into a service for
better testing and isolation.

Pass bridge to SubscribeBridgeService

Move bridge subscription to Pipeline processing

Moves the bridge subscription mechanism from the Pipeline creation chain
to the Pipeline processing service.

Rename dependency to upstream

Renames dependency to upstream.

We already have dependencies, using dependency would be confusing.

Refactor subscription services

Refactors subscription services to better encapsulate the business logic
they enforce.

Move bridge subscription to enqueue event

Moves the bridge subscription logic from the ProcessPipelineService into
the enqueue event.

Parse upstream bridges in pipeline seed

Add optimistic lock to bridge subscription service

Add upstream mirror docs

Mirror completed statuses to bridge

Rename upstream to needs:pipeline

Renames the upstream CI config keyword to needs:pipeline.

This supports the idea of marrying DAGs and upstream dependent
pipelines.

Extract upstream_pipeline to method

Rename needs:pipeline to needs:project

Improve 'needs:' docs