If xaxis is datetime, auto-change it to datetime

[alison985]

Right now the xaxis is assumed to be a datetime measure. If it is not you have to manually change it to something like "category". This is mildly annoying. We should be able to detect the xaxis type and auto-change it for the user.

[alison985]

Pausing this for a bit as it's become larger than I originally thought. It requires referring to the actual chart data within the chart editor which hasn't been there before.

[alison985]

I did a status commit of where I left off on this in this branch: https://github.com/alison985/mozilla-redash/tree/228_auto_pick_xaxis_type