Promote the GitLab.com Gold trial on a users default dashboard

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22901

Useful for reviewer

To see the callout again after you've dismissed it:

psql -h $(pwd)/postgresql -d gitlabhq_development -c "DELETE FROM user_callouts WHERE id > 0"

To mock enable a trial on the H5bp group, which the root users owns by default:

psql -h $(pwd)/postgresql -d gitlabhq_development -c "UPDATE namespaces SET trial_ends_on = '2020-02-02' WHERE name = 'H5bp'"

To remove the trial mock, you can run the same command but change trial_ends_on = '2020... to trial_ends_on = '2000....

To mock enable a gold plan on the H5bp groups:

psql -h $(pwd)/postgresql -d gitlabhq_development -c "INSERT INTO plans VALUES (1, current_date, current_date, 'gold','gold', 1000, 1000)"

psql -h $(pwd)/postgresql -d gitlabhq_development -c "UPDATE namespaces SET plan_id = 1 WHERE name = 'H5bp';"

To remove the gold plan mock, you can run the last command, but change plan_id = 1 to plan_id = null.

What does this MR do?

Adds a callout/banner to GitLab

• • Dismissable banner (saved to db)
• • Display on all types of /dashboard.
  • • Projects
  • • Starred projects
  • • Groups
  • • Activity
  • • Starred activity
  • • Issues
  • • MRs
  • • Todos
• • Only enabled for GitLab.com (and dev)
• Do not show to users who;
  • • Have dismissed the banner
  • • Are on a trial or who have already done a trial
  • • Are already on Gold

Screenshots

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #6983 (closed)