add api adapter / backend link behavior.

Re: https://github.com/tastejs/todomvc/pull/1050

Modifies link display and behavior after changes from the above PR are merged. The PR also has logic changes to the template from learn.json, in a backward compatible way: https://github.com/tastejs/todomvc/pull/1050/files#diff-719bcdeda5b5e8046f18065efe042e27R2273

Version bump after this would be 0.3.0? If 0.2.1, I'll have to change https://github.com/tastejs/todomvc/pull/1050/files#diff-75be9f28ab6ec31d1dd17ce7c7600e47R6