This replaces the `{{#previous}}` and `{{#next}}` handelbars helpers
with simple objects that contain the previous and next values. These
helpers have been a bit fussy to work with and have caused issues in the
past. This drops a large amount of somewhat fragile code with something
that is a bit simpler.
Additionally, this switches the previous/next arrows to use an `{{#if}}`
instead CSS trickery which may help with upcoming changes to
font-awesome.
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