User can't use a preview mode with a large text with a big amount af bb-code tags

Precondition
User should be registered and logged in on forum.

Steps to reproduce:

1. Create a new topic in some branch by using "New topic" button.
2. Paste a text from an attachment (Stas test text.txt) to a topic field.
3. Name the topic and click "Preview".

AR: warning "size must be between 2 and 20000" shown.
ER: topic opened in preview mode and formatted text should be shown.

Notation: if you click "Save" button topic created and formatted text should be accessible.