Provide link to problem on Project Euler for each problem; use /problem/:number URI

Note. Previously the URI was /problem/:id but params[:id] was used
as the problem number not its id in the database. It is probably a
mistake to have an automatic id in this table; the primary key should
be the problem number.
Display Project Euler problem URL as link text
Clarify what Rankings/Manage Event refer to by including event name
Demote 'Sign up and RSVP' from h2 to p
Add links to problem numbers in rankings; allow for "equal 1st"
Remove rankings, manage links from navbar; restore manage link to events#index
Except events#show from using .container and its margins
Observe finalized attribute of event
Show teams that have solved a problem against each problem
Restrict management of events to event creator (admin)
Correct routes.rb to use the singular number for an individual event
Give subquery an alias (required by Postgres)
Display one of "belongs to ...", "finalized", "Manage"