Panel improvements: ‘Filter cells in dataset’

[jeremymiller]

• Add a grep filter (e.g. all subclasses with STR rather than needing to manually type 10 separate ones in the filter).
• Add a negative filter (e.g., everything EXCEPT cell_type_x).
• Add “Go” button to the filter rather than having it auto-update to avoid it refreshing every time you change anything. This will be important when trying to do a lot of filtering on big data sets.
• Improve documentation for filtering by more than one thing.
• Show distribution of numeric variables for numeric filtering: Tain to share code ==> EDIT FOR THIS ONE: code to build a histogram is already included in server.R (near lines 760-780), but it doesn't properly refresh when the slider refreshes for some reason that I've yet to figure out.