refactor conditional caller fork in shifts index view
add pagination to shifts index
Add format_start and format_end to ApplicationHelper, add spec for pagination
add sortable columns to shifts index (root path)
refactor shifts index - move start & end columns into time column
tweak css for table row headers, add up & down arrow images
extend sorting to rider shift lists and restaurant shift lists
refactor Shifts#load_shifts -- add associations &  caller_filter methods, move redirect_non_staffers to separate before filter
add specs for sorting
Add date filter to shifts index
Add specs for date filters on shifts index
Add rider filter to shifts index
Add restaurant and status filters to shifts index
refactor shift index filters, add filters to rider and restaurant shift indexes
refactor Assignment model -- allow intialization without rider_id & add :unassigned status
add new assignment logic to factories and sample data
add new assignment logic to ShiftsController
add new assignment logic to Shift#assigned? and Shift#assign_to
add new assignment logic to assignment views
Add new assignment logic to shifts index