Merge HTTP and Keyword monitor types, or enable easy conversion#303

In abstract, the Keyword monitor type is just the HTTP monitor type with a keyword added. In fact I often find myself creating an HTTP monitor first and then later wanting to add a keyword to it. But there is no way to add a keyword or convert the monitor type. So you have to create a whole new monitor from scratch.

Right now:
(1) HTTP monitor is a ‘base’ HTTP monitor with status code checking added.
(2) Keyword monitor is ‘base’ HTTP monitor with keyword checking added.

However it would a valid use case to check both status code and keyword at same time, e.g. to verify the correct error message is being returned.

If you just add status codes support to the Keyword monitor, I think the HTTP monitor can be deprecated. Or vice versa.

Thanks for the suggestin @Aaron Roydhouse