Many places where $bp->root_domain is more appropriate than site_url()

[johnbillion]

Many template functions such as bp_search_form() use site_url() to print the site URL. The problem is that site_url() returns the URL of the current blog, not the main site URL.

For example, if you use bp_search_form() to print the BP search form in a member blog theme, the form action will point to the current blog and not the main site, and will produce an error when submitted.

If $bp->root_domain was used instead this would allow these template functions to be used within member blog themes.

Note that some template functions such as bp_signup_page() already use $bp->root_domain where others use site_url().

Patch coming in a lil' while.

[johnbillion]

Patch which also includes replacing some occurances of get_option('home') with $bp->root_domain to allow these functions to be used within member blog themes.

[apeatling]

(In [1254]) Fixes #606