API keys for authentication, API-centric experiments depending only on tagging-common

[imalsogreg]

After finishing the API-key login, it would be nice to have experiments that only depend on tagging-common, and be hosted somewhere other than the tagging-server. e.g. it should be possible to give a subject an API key and a url for the server, and have the user plug those into some fields in a frontend hosted on github pages.

It's still a little unclear where/when the tagging user accounts and API keys should be generated under different conditions (pre-selected pools of subjects vs. amazon turk workers).

[imalsogreg]

For instance, drawcopy would benefit from being able to draw stimuli from and send results to a tagging server, but it would be unfortunate to be forced to version-control drawcopy alongside the rest of tagging.