Check cache timeout settings

Description

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/33501/

"static assets should be aggressively cached. Avatars, for example have a 5 minute cache timeout which is very short considering how many of them we're loading on GitLab and how infrequently they change."

Proposal

• Investigate the cache timeout settings for static assets, set a guideline to follow and increase the timeout where reasonable according to the guideline.

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)