Add local markdown version

What does this MR do?

Adds local markdow version. This version is stored in DB (application settings) and it can be bumped by admins when necessary to enforce re-rendering cached texts.

To assure that markdown cache is invalidated when either of hardcoded or local markdown versions are increased, final cached version is now combined (bitshifted) from both.

cached_markdown_version column in DB has 4 bytes (integer), supposing that both hardcoded and local versions are 2 bytes, we can keep using the existing column - for this reason local_markdown_version is limited to to 0-65535.

omnibus-gitlab!3031 (merged) adds a note about this to external_url on omnibus side.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32789

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer