add basic admin ui for setting the shortlink base
add basic admin ui for setting the shortlink base

The shortlink base URL is set on the general settings page.  I'm using a
bit of javascript to move it up alongside the other URL settings which
results in a slight visual jump, which kinda sucks.

This change also allows defining the shortlink base using the constant
'HUM_SHORTLINK_BASE'.

willnorris/wordpress-hum,willnorris/wordpress-hum