FEATURE: allow for a larger maximum post length

In some documentation heavy sites larger posts may be desirable.

This is not a setting to change without thinking, however where it is
needed it can make sense. Allow for a 50% increase in max over our
previous maximum.

The old limit of 99000 is arbitrary. Stuff gets riskier the longer posts
get especially with complex markdown, however 99000 is not a threshold
where stuff automatically breaks.

config/site_settings.yml

@@ -637,7 +637,7 @@ posting:
   max_post_length:
     client: true
     default: 32000
-    max: 99000
+    max: 150000
   topic_featured_link_enabled:
     client: true
     default: true