Remove strip viewer, keep strip view

[SmilyOrg]

This is about the strip viewer added in #47. The general idea is good - common left-right swiping when zoomed in, shown on a black background. However, the implementation leaves a bit to be desired.

Problems

• It has visual glitches as the two separate viewers (scroll view & strip view) need to be reconciled switching between them and this takes a lot of effort to solve
• It's not efficient as the same photos are loaded in a separate view, especially right now as both views load at the same time and only one is shown
• The strip viewer might be running into numerical issues as 300k photos are put next to each other... or at least it leads to very long virtual images. Not necessarily a problem, but another potential issue down the road

Solution

Potentially go back to the previous solution, where the existing views (scroll viewer, map viewer) are zoomed into directly, reusing them to switch between photos. Here are some problems from last time and how they can be addressed this time.

• Nearby photos are visible and not hidden under a black background. This can be solved by hooking into OpenLayers rendering directly and masking out everything that is not the desired region. This is the approach tested in https://github.com/SmilyOrg/photofield/tree/remove-strip-viewer
• Switching between photos is problematic as they are not arranged linearly on a horizontal strip. This can be resolved by virtualizing the movement with the viewer always showing the focused region, but the viewer itself being moved left and right or up/down.