Reward copy refactor

I recently made changes to the copy in the reward pane: https://github.com/stellar/stellar-client/pull/775

This new PR refactors the stuff from PR775 and unifies the getters for the copy in the reward pane.

Main things:
- Templates now access a unified `$scope.reward.getCopy()` to get either the title or the subtitle. Before, it was direct access to scope variables set every time it received an update from fbgive.