Rename base model to avoid Rails getting the wrong ApplicationRecord

What does this MR do?

Fixes an issue with !64289 (merged), where all models under Ci inadvertently inherited from Ci::ApplicationRecord, when we only want Ci::InstanceVariable to do so.

No changelog because this feature is still in development, and only occurs when the ci db is configured in config/database.yml

The error:

Related issue: #333527 (closed)