Update availability indication

### Summary

It would be convenient to list whether the sentry on-premise or sentry client versions have updates available.

### Motivation

Sometimes site updates get missed. The current process for me is to check `composer outdated` on each code base, and occasionally visit GitHub to see if there is a new sentry on-premise release.

It may be to the benefit of the sentry community to have more people testing the latest releases, and especially if there are security or stability fixes that they would benefit from.

### Additional Context

I would envision this to be something like what is available in gitlab when visiting the "admin tools" section, where current versions are listed, with indications of updated versions.

Gitlab wouldn't have an indication of "client library versions", so this would be an extension of that concept.