Add Conflicts#new, Conflicts#edit, Conflicts#delete, add conflicts teaser to Rider profile, refactor Shift table teasers
cleanup conflict request specs
Test relative links for Conflicts with different callers (postpone full implementation til later)
add start and end to Conflict model
add start and end to Conflict model
clean up Shift and Conflict specs
Refactor Conflict views to match revised data model (with start and end fields)
Add Rider.assignments_on(date), Rider.conflicts_on(date), Assignment.conflicts_with?(conflicts), Assignment.double_books_with?(assignments)
Add spec for Assignment.double_books_with?
Add full set of test cases to specs for .conflicts_with? and .double_books_with?
Move .conflicts_with? and .double_books_with? from Assignment model to Shift model
update Assignments controller to reflect new conflict-checking methods on Shift model