OPML import file size limit

[yoavf]

The opml import script currently uses a hidden input field of "MAX_FILE_SIZE" to limit the import size (currently 30000 bytes)

1. This is too small
   
2. This is a hidden input field that can overridden with tools like Firebug etc.

Suggestions

1. Either remove the file size limit and rely on each server file limit.
   
2. OR up the file limit (100kb at least) and enforce that in the php script

I can submit a patch for option 2 if that's better.

[sivel]

Use wp_max_upload_size and fix a notice for undefined index.

[nacin]

(In [13306]) Fix notice, and use wp_max_upload_size() to fill MAX_FILE_SIZE hidden field (used by supporting browsers for a client-side check). Fixes #10849 props sivel.