make the site rebuild automatically

[ralsina]

Looks like ghostjs can now do decent screenshots, so I could do this in the VPS.

[Kwpolska]

To save resources, you could use a webhook and rebuild only when necessary™, for both themes and plugins (and maybe even automate nikola-site?)

• Webhooks docs
• Sample implementation of the issues hook in Twisted by yours truly, complete with signing
• push docs