Disable some highlighting for MOUSs

(...modules of unusual size.)

Enabling full syntax highlighting for very long Lua modules can produce DOMs
that have hundreds of thousands of elements and cause browsers to lock up.

I took a count of spans by class (which amounts to a count of tokens by type)
of https://en.wiktionary.org/wiki/Module:languages and came up with:

       sy0:      62545 (symbols)
       br0:      61952 (brackets)
       st0:      39291 (strings)
       kw3:       7746 (keywords)
       kw1:          3
       kw2:          2
       co2:          2
       co1:          2
       nu0:          1
     ------     ------
     Total:     171544

GeSHi allows you to disable highlighting for a particular token type (see
<http://qbnz.com/highlighter/geshi-doc.html#disabling-lexics>) which like a
good way of handling this issue.

Disabling symbols (set_symbols_highlighting(false)) removes both sy0 and br0
elements from the DOM (about 124k elements in the case of Module:languages),
with about 47k elements remaining on the page. This is enough to make Chromium
responsive on my laptop (2.3ghz i5, 8 GB RAM), but it's still noticeably
sluggish. Adding 'set_string_highlighting(false);' removes another 40k elements
from the rendered output, and the resulting DOM is quite zippy at 8k elements.

Proposed solution: disable symbols highlighting when >100 kB; disable strings
highlighting too when >200 kB.

Change-Id: I90c645f9d03bbdc135058a3717a463dec40aa77d