Support field filter variable types

[tonysimpson-sonocent]

We'd like to be able to export and import Cards with field filters (aka template_tag of type "dimension") these are used to parameterise queries e.g. WHERE {{date}}.

We're going to take a look at doing this ourselves, we're assuming we can workout the stuff that isn't in the Metabase API docs from the Metabase code.

[JoelDellamaggiore]

would be great what Tony suggests, because then when importing, if the query has that WHERE {{date}} filter, it would not break, currently if it does break.