Change verbage displayed at templates.php to express sets

- Change the table header from `Email Templates` to
  `Email Template Sets`
- Change `Add New Template` to `Add New Template Set`

This removes any possible confusion for new users. Instead of
thinking that they have one template because it displays one
template set, the new verbage will clarify that the table
at templates.php is a list of their template sets.
Merge pull request #841 from beckonandmuse/issue/template-verbage

Change verbage displayed at templates.php to express 'sets'

Reviewed-By: Jared Hancock <jared@osticket.com>