Fix rerun cucumber profile.

For some reason, it would not pick up the step definitions unless an
extra `-r features` was added to the arguments.
Install gem 'pry-stack_explorer'
Rename `SubEvent#compute_placements`

To `SubEvent#compute_sub_placements`
Sort placement table by couple number, not rank

Since there cannot be ties in the couple number (unlike with the rank)
this is guaranteed to be consistent.
Auto-increment the `order` of SubEvent factories
Make `has_many :through` associations distinct

It is an error for a couple to be associated with a single round
multiple times.
Use `schema_plus` gem to create db view.

This allows adding and dropping the view to show up in  "db/schema.rb",
which lets "rake db:schema:load" work properly. "db:schema:load" only
runs the "db/schema.rb" file, and using `execute` in a migration does
not get added to "db/schema.rb".

`schema_plus` also tells the database to enforce foreign-key
constraints, so the order of seeding has to be changed so that `dances`
are added after `sections`. The `seed_fu` gem loads the files in
alphabetical order, so renaming the files guarantees the proper order.
Only assign rule 10 if no rule is already present

This way, when rule 11 calls back to rule 10, the fact that rule 11 was
needed to break a rule-10 tie is not lost.
Feature syntax changes.

For all steps which take tables, add a colon at the end.

Add tags to all features and scenarios.

Modify `the placement of the couples should be:` to expect rule
specifier in separate column, which is optional.

Add some examples from real-world competitions.