Move tags + taggings to be on CI database

Move tags + taggings to be on CI database since it is 99% of time used by CI

According to #21399 (comment 623468507), 99% of rows in taggings is CI. The only other non-CI is Project, used for Project.topics.

Proposal

• Move tags + taggings table to CI database
• Ban further use of acts_as_taggable* / acts_as_ordered_taggable*
• Make acts_as_taggable to use CI database and CI model
• Project
  • Implement new project_topics table, stop using acts-as-taggable => #335946 (closed)
• Issue / MergeRequest
  • Truncate old data => #335794 (closed)