Signatures have html entities when merging from phpBB3

[TK-3375]

When merging phpBB3 into MyBB 1.8 I noticed user signatures get corrupted, here is an example :

<t>I am ready to serve the Empire !<br/>
something something</t>

All kinds of html stuff is weirdly converted from old phpBB BBcodes used. If I may add : that "<t> </t>" thing also shows up in forum descriptions, and in forum rules. Maybe those fields could use some cleanup as well.