Improve JSON button

[arildm]

The current JSON button reflects a root-scope variable that is modified when switching between result tabs. When a request is made, the url is stored (prevUrl) so that the corresponding tab can then copy it to the root-scope variable (jsonUrl).

There are some drawbacks to this:

• Result tab controllers must have enter/exit handlers
• Requests with many large parameters will use POST instead of GET, and then the url is not enough to reproduce the request

I think we should use the full request conf instead (or just url+params), and output the button in each tab rather than outside them.