When retrieving a user's networks, global admins see all

See #160
Introduce WSUWP_COOKIE_DOMAIN constant for wider auth ability

Build the cookie domain based on the configuration of the WSUWP_COOKIE_DOMAIN
constant. If it is not set, we set cookies to the originally requested domain.
If it is set as auto, we find the root domain of the request and use that for
cookies. If it is set and not explicitly defined as 'auto', we assume that a
cookie domain is being specified, similar to how you would do when using
COOKIE_DOMAIN originally.

See #162
When using the Add Existing user option, add user to network

Closes #163
Remove dated todo from "My Networks" addition
Merge pull request #161 from washingtonstateuniversity/fix/networks-menu

[PR] Corrections to networks menu, login, and cookie behavior