UI for sampling

[benjeffery]

Allow the user to enter and control the sampling of a table in the following contexts:

• Non-histogram plots (Histogram plots covered by Histogram type plots should have binning done in DB, not on client. #734)
• Genotype viewer rows (I don't think columns would make sense)

1.6.2 allowed sampling in table views - but I don't really get how that is useful.

[leehart]

For posterity: sampling in table views has a bearing on rows pagination, which will need to fetch and show the relevant page of random rows from the full (fixed) set of random rows, where the number of rows has been specified by the user, as per v1.6.2.