Added 'activityTerm', 'stepTermPlural', and 'activityTermPlural' as project variables (which will be authorable)

- Added view utility functions which return the step and activity terms (singular or plural) to vle dialogs and messages - they return the default terms for 'step' and 'activity' as specified by i18n files if terms are either not set for a project or are invalid (all whitespace strings, for example)
- Added stepTermPlural authoring
Merge branch 'master' of https://github.com/WISE-Community/WISE-VLE