Group election candidates by region; open statements from ballot

[kjaymiller]

• Election page now groups candidates by continent and subregion, using the same group_by_continent grouping as the ballot.
• On the ballot, clicking a candidate's name opens their statement in the same modal as the election page. The modal markup moved to a shared include (includes/candidacy_statement_modal.html).
• The rank input gets an aria-label ("Rank for ") since the name is no longer a <label>.
• Ballot candidates now select_related the leader profile for the modal.

Tests added for both the grouping and the ballot modal.

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