R1:7e37eb48273e

Provide a highlighter cache for Paste

Summary:
D4188 adds a preview of Paste contents to the list, but gets slow for large lists if you have Pygments installed since it has to spend ~200ms/item highlighting them. Instead, cache the highlighted output.

- Adds a Paste highlighting cache. This uses RemarkupCache because it has automatic GC and we don't have a more generalized cache for the moment.
- Uses the Paste cache on paste list and paste detail.
- Adds a little padding to the summary.
- Adds ".