Move checks if import source is enabled to service layer

GitLab has multiple ways of initiating same action. At least 3 are known to me:

• UI (controllers)
• REST API
• GraphQL API

ATM, check that import source is enabled is performend on "controllers" levels (or their counterparts for REST/GraphQL) This results in issues like #340782 (closed) and #29748 (closed)

In order to avoid future issues it is important to move relevant checks to service layer