Make the "kite offline" iframe content URL configurable

[BjarniRunar]

Currently the URL for the "fancy" version of the "kite offline" error page is hard coded. It should be configurable at runtime.

[guillerodriguez]

There is now an option to make the "offline" url configurable. However even if this is used, there are still mentions of pagekite in the HTML boilerplate. You may consider to change this for consistency with the way it is currently done in PyPagekite:

# Do we have a more specific error URL for this domain? This is a
# white-label feature, for folks not wanting to hit the PageKite.net
# servers at all. In case of a match, we also disable mention of
# PageKite itself in the HTML boilerplate.

https://github.com/pagekite/PyPagekite/blob/602dc67d5626ef38d4013ece52bcf54eb8522f9e/pagekite/proto/selectables.py#L601

[BjarniRunar]

Agreed, leaving this open until that's done.