[FIX] fixes issues with custom filters and search view

The problem was that when the user manipulates the graph view (in pivot
table mode), the graph view resetted the group by facet in the search
view.  It was not a problem unless a custom filter with a groupby was
already there, in which case, the group bys were duplicated.

The search view is now smarter, it only resets the additional groupbys
(and col_groupbys).   Also, to prevent usability problems, the graph
view disable the '+/-' groupbys added by a custom filters.

Note that this fix is only temporary: a revamp of custom filters, facets,
search view is coming in the next months. (at least, that's the idea). Right
now, too much 'search logic' is in the graph view.

Another note: this fix is somewhat fragile: it makes some assumptions
about the search query (mainly that the custom filter is the first facet,
also, that no other filters add groupbys/col_groupbys)
* implicit globals?
* I don't understand the purpose of `slice` here, it takes the start index so that looks like a complex way to create empty arrays