PageKite UI module

Hello,

I have implemented the first cut of the PageKite UI for Plinth to seek feedback. I have also made some minor fixes in Plinth required for it to work. Currently:

- It only works for HTTP and SSH services
- Only pagekite.net accounts are supported (future plan is support other FreedomBoxes)

I intend to continue working on it. Please merge these changes as they are useful themselves and also so that I can request feedback.

Thank you,

-- 
Sunil