Zoom fixes

[blepabyte]

Fixes #1504

Contributor checklist:

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've squash'ed or fixup'ed junk commits with git-rebase
• I've built the docs and confirmed these changes don't cause new errors

Proposed changes

• The mousewheel and DOMMouseScroll events are deprecated (wheelDelta was undefined causing the NaN's) so I swapped to the wheel event, which has better browser support.
• Made some changes to address the docs issue in Zooming in IJulia notebooks #1272 - I built the docs and zoom is now working on all the browsers I tested (additionally, the co-ordinates are now correct)
  • Plotting from REPL worked since the plot was always at the exact top-left corner of the page

This does not completely fix the original IJulia notebook problem in the linked issue (when shift left-click + dragging to zoom) as jupyter seems to do something weird with mouse events.

[bjarthur]

thanks!