Input filter support for activity messages

As activity is going in the direction that allows users to create their own custom triggers some users will want to use html in their activity messages. I just ran into this limitation by trying to use something like.

You have logged in
<a href="[account-url]">[user-name]<a /> has logged in.

Which looks like this in views.

You have logged in
<a href="/users/guest">guest<a /> has logged in.

This is tested under 'user' activity and 'login' trigger.