[Explore]set default aggregate based on selected column type

[mistercrunch]

We could save 2 clicks by assuming people want to SUM numeric columns and COUNT(DISTINCT {col}) strings.

[junlincc]

@mistercrunch This popover is gonna function quite differently in d&d, we will take it into consideration while redesigning.

[junlincc]

set dynamic aggregate function default based on column types
numeric and contains _id_ _key_ _code_ -> COUNT(DISTINCT)
other numeric -> SUM
strings -> COUNT(DISTINCT)
TEXT -> SUM(LENGTH) (?)

@zhaoyongjie @villebro