Asset optimization

[jesseleite]

Though there is still room for performance optimizations, this should fix timeouts in asset browser when working with thousands of files (both locally and in S3).

Closes #2145
Closes #1188

Also possibly related...?

References #2644
References #2279

[jasonvarga]

More details:

• The asset meta data gets cached to prevent on extra file reads (API calls on S3, DO, etc) and YAML parsing.
• Folder listings get cached. An API call to S3 to get the contents is what takes a chunk of time.
• We're only converting paths to assets (and getting meta data) for what's on the paginated page, where before we were doing it for everything in the folder. That's where the main slow down was.
• As soon as you add a where clause or orderBy, we need to convert all the paths to assets, so it'll be slow again. In the asset browser, we don't do either of those things, so we're good.
• If you perform a search, it would be a where clause, so it'll be slow. The workaround (which was already a thing you'd want to be doing anyway) is to have an actual search index containing the assets. That's fast.