This commit extends the `compute_placement()` function in
`popovers.js` to take into account height/width of popover as well as
positioning preference. If vertical positioning is desired and the
popover fits in either 'top/bottom' positions then we don't check for
`left/right' positions. Earlier the behavior was to prefer
'left/right'positions over 'top/bottom' positions, which resulted in
the emoji picker popping incorrectly to the left.
compute_placement utilizes the dimensions of the viewport, viewport location of
an element, and dimensions of an element to determine if a popover will fit
horizontally and/or veritically given its orientation. The default placement
is now viewport_center, which displays the popover, without an arrow, in the
fixed center of the viewport.
This should be particularly useful for hotspots on mobile or large popovers
that contain a lot of content. The property hotspot.location.popover can be
optionally set to fix the orientation of a popover (most likely to
VIEWPORT_CENTER).
This commit adds the backend support for a new style of tutorial which
allows for highlighting of multiple areas of the page with hotspots that
disappear when clicked by the user.
