Fix the position of the popup menu after rubberband select when there is a margin on the component e.g. in embedded

[cmdcolin]

Currently the embedded mode, most clearly, can have an example where the "Zoom to region" menu after a rubberband selection is shown in the wrong place

To properly place the menu using anchorPosition it should use left: event.clientX, top: event.clientY from the mouseup event (these are screen coordinates instead of offset to any particular element)

Found while looking at the storybook :)

[codecov]

Codecov Report

Merging #1747 (efab8c7) into master (49862a1) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1747      +/-   ##
==========================================
- Coverage   58.81%   58.80%   -0.02%     
==========================================
  Files         454      454              
  Lines       20999    21001       +2     
  Branches     4974     4974              
==========================================
- Hits        12351    12350       -1     
- Misses       8338     8341       +3     
  Partials      310      310              

Impacted Files	Coverage Δ
...iew/src/LinearGenomeView/components/RubberBand.tsx	85.59% <100.00%> (+0.24%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	89.36% <0.00%> (-4.26%)	⬇️
packages/core/util/index.ts	82.20% <0.00%> (-0.33%)	⬇️

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[cmdcolin]

This diff can help test things like this in the main app

diff --git a/packages/core/ui/ViewContainer.tsx b/packages/core/ui/ViewContainer.tsx
index 57fc28962..ca31964c3 100644
--- a/packages/core/ui/ViewContainer.tsx
+++ b/packages/core/ui/ViewContainer.tsx
@@ -20,7 +20,7 @@ const useStyles = makeStyles(theme => ({
   viewContainer: {
     overflow: 'hidden',
     background: theme.palette.secondary.main,
-    margin: theme.spacing(0.5),
+    margin: theme.spacing(50),
   },
   icon: {