Enable customizable quota-related error messages

[paulproteus]

Context:

• When being used by a hosting company, Sandstorm needs a way to show a message for different quota-related states, e.g. User is nearly over quota; user is over quota. A hosting company would want to customize this message to e.g. include a link to their billing control panel, such as WHMCS.

Proposal:

• The hosting company should create a web page in e.g. Hacker CMS for each state, and that Sandstorm would have an admin settings feature that allows you to input a URL that we embed via an IFRAME, one different URL per quota state. See https://docs.sandstorm.io/en/latest/administering/faq/#can-i-customize-the-root-page-of-my-sandstorm-install for an example of doing this now.
• Sandstorm should provide an admin interface to enable the admin to choose which URLs are being IFRAME'd.

Other remarks:

• Is there a way we can show the admin a preview of these messages in context? I imagine that would be useful.

Implementation details:

• This requires moving/copying some oasis.sandstorm.io-specific templates into Sandstorm, whereas right now they're in the blackrock repo.

[fgiorlando]

Sounds like a good solution,

The link in WHMCS could go directly to the "product" required, eg. increase quota or upgrade account. More details on WHMCS linking here for context: http://docs.whmcs.com/Linking_to_WHMCS