R1:21826ed7b35a

Don't highlight very large files by default

Summary:
Ref T5644. See some discussion in D8040.

When a file is very large (more than 64KB of text), don't activate syntax highlighting by default. This should prevent us from wasting resources running `pygmentize` on enormous files.

Users who want the file highlighted can still select "Highlight As...".

The tricky part of this diff is separating the headers into "changeset" headers and "undershield" (rendering) headers. Specifically, a file might have these…