add GET api call to `/api/<accountId>/support-url`

[inztinkt]

In ref to: https://github.com/trustedlogin/trustedlogin-ecommerce/issues/196

• check for an updated supportUrl value via TL SaaS
• allow SaaS to know why we're redirecting
• if no result, use setting in config object

[inztinkt]

@zackkatz for this one, some of our 'redirects' are happening in JS when the SaaS/etc fails and a button is clicked.

So we can:

• have the JS ping the reason to SaaS directly via a $.post when the buttons are clicked
• or have the JS ping ajaxurl on Client site and have PHP ping SaaS when the button is clicked
• or have the JS ping a REST api endpoint on Client site and have PHP ping SaaS when button is clicked

Any preference?

I believe they're ordered in increasing amount of security.

[zackkatz]

Let's do the REST API endpoint @inztinkt