Channel provider for AlphaBrowse index

[demiankatz]

This PR adds a new channel provider to find nearby records in the alphabrowse index; most useful for displaying "nearby on shelf" information.

TODO

• Get thumbnail images working
• Consider hiding the "seed" record from the browse list Leaving it in place for now; provides some useful context.
• Eliminate unnecessary code copied from SimilarItems provider (used as foundation for this)
• Make sure all labels and cross-links are appropriate for the context; add new translation strings if necessary
• Test that all configuration settings work properly (LCC vs. Dewey, etc., etc.)
• Make sure channel tokens work in a sensible way
• Consider an "infinite scrolling" mechanism to allow the browse to continue indefinitely in both directions Could be useful, but that significantly broadens the scope of this PR; can consider later.
• Test that functionality behaves correctly when no valid call number is found
• Improve handling of adding empty channels (currently it causes an empty channel to appear -- it might be better to handle this in a different way; note that this may be a more general problem that needs to be fixed in master independently of this PR).