Move "getPluralPhrase" method from "PageHelper" into "kMultiLanguageHelper" class

In In-Portal 5.2.0 we introduced page content revision control (see #001117 task).

One of a goals in this task was to display number of other users, who are also editing current page alongside with current user.
Warning message would be different based on user count:

• 1 user is editing
• 2 users are editing
• 5 users are editing

In English language text (not including number) is the same for 2 and 5 users, but in Russian and most other languages they differ.

To display correct words near user count we crated PageHelper::getPluralPhrase method.

I think that this method deserve better visibility for developer and I'm proposing to move it (and related methods) to kMultiLanguageHelper class.