Allow dates formatted as YYYYMMDD to be displayed as a datetime

[mreid-moz]

Issue Summary

Mozilla uses YYYYMMDD date format in a bunch of places, primarily in our representation of "submission date". When used on the x-axis of a chart, the system defaults to interpreting it as a datetime (see #228). The resulting chart almost works, but requires a change to categorical before data will be displayed properly. In contrast to #228, we could actually interpret these values as dates to make things work with the default settings, we would need to detect and parse them differently.

Steps to Reproduce

1. Create a query with a YYYYMMDD-formatted date field (example here)
2. Create a chart with this field on the X-axis
3. Observe the broken date handling when YYYYMMDD strings are treated as Datetime (the default)
4. Change the X-axis scale to "category"
5. Observe that the X-axis looks normal.

Note the current workaround is to re-format the data as "YYYY-MM-DD", which is interpreted as expected.