WIP: Split Ci:Runner to 2 STI models

What does this MR do?

Split the Ci::Runner model into 2 STI models Ci::InstanceRunner and Ci::ProjectRunner, while removing the boolean flag attribute is_shared.

• Shared runners are now instance runners
• Specific runners are now project runners

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This is groundwork for the implementation of group shared runners https://gitlab.com/gitlab-org/gitlab-ce/issues/10244.

According to the discussion in https://gitlab.com/gitlab-org/gitlab-ce/issues/10244#note_24927551 this solution was the requested way to implement the runners.

The development of this MR is sponsored by @siemens (/cc @bufferoverflow).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/10244