Fixes #120 - dynamic steward pagination

- Makes pagination of Stewards in notifications view dynamic so it says
“1 of 3 Stewards,” “2 of 3 Stewards,” etc. when navigating between
agencies.
- Amends next/previous method names related to stewards to include
“Steward,” such as “canNext()” becomes “canNextSteward().” This makes
it clearer what the methods do and aligns them with the names given to
the trail methods that perform similar actions.
- Adds `currentStewardPosition` method for retrieving the current index
position of the stewards array. This is used to update the stewards
pagination control with the currently shown steward position in the
list of total stewards.
Merge pull request #123 from codeforamerica/120-dynamic-steward-pagination

Fixes #120 - dynamic steward pagination