insights: save settings for chart starting at 0 vs scaled y-axis

Created by: Joelkw

We should save the settings for using a scaled y-axis (or starting it at 0) if a user changes from the default. We already save filters, so to reduce complexity, let's use the same model (anywhere a filter shows up – for all users of that insight – the y-axis setting should persist as well).

Valid feedback here from a user at https://github.com/sourcegraph/accounts/issues/581:

Code Insights: why don't chart y axis default to start at 0? Or at least persist when I explicitly tell it to