Add LocalMarkdownVersion field to Settings and UpdateSettingsOptions.
Incrementing LocalMarkdownVersion invalidates GitLab's internal
Markdown cache, which is helpful in certain situations.  See:

https://docs.gitlab.com/ce/administration/invalidate_markdown_cache.html
https://docs.gitlab.com/ce/api/settings.html#get-current-application-settings
https://docs.gitlab.com/ce/api/settings.html#change-application-settings