Reflow output: 'wsl_render_login_form' (shortcode)

Reflow output of 'wsl_render_login_form' to correct for bad behavior
when using shortcode. A page including shortcode
[wordpress_social_login] would render at the very beginning of the
container page, regardless of where the shortcode tag was placed.

All output from 'wsl_render_login_form' is now buffered and returned
rather then being immediately written to the document. This allows for
proper output at the correct time. Particularly:
- do_action: This function does not return anything, all required output
to document should be done before termination.
- shortcode: Output should be parsed and then written to correct
position in document, replacing [shortcode] with content at same
location.

P.S. This is my first code commit, apologies if I have incorrectly
committed this code.
Fix: Reflow output for 'wsl_render_login_form'

- (major) Added 'echo' correctly to all other 'do_action' calls to
'wsl_render_login_form'
- (minor) Combined 2 lines of code into one, removed unneeded variable
instantiation.