Add support for application settings cache expiry config

What does this MR do?

Adds support for making application settings cache expiry value configurable.

• Related Issue: gitlab-org/gitlab#217010 (comment 484198651)
• Related GitLab MR: gitlab-org/gitlab!51889 (merged)
• Related Omnibus MR: gitlab-org/omnibus-gitlab!4938 (merged)
• Related documentation MR: gitlab-org/gitlab!52314 (merged)

Related issues

gitlab-org/gitlab#217010 (comment 484198651)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened