Deduplicate tag_list in Ci::JobDefinition

Problem

ci_build_tags_* partitions are among the heavy users of disk space. source

Solution

As we now have Ci::JobDefinition model, we can deduplicate tag_list into the definition.

This issue consists in persisting tag_list into job definitions. Later in Phase 3 we will migrate data from ci_build_tags_* into ci_job_definitions.config[tag_list].