Add a filter to bp_dtheme_firstname() in BP Default theme

[r-a-y]

For those of us using an xprofile for first name or for anyone who might want to override this function, adding a filter would be great!

I know we could easily duplicate the post-form.php file into our child theme, but I don't think we should do this just for a string.

[apeatling]

(In [2629]) Fixes #1814