Refactor duplicate definitions of replacement variables

[igorschoester]

In PHP the replacement variables are determined. They are used for the meta values on a page. We pass these along to JS and use them in the snippet editor (UI for the user).
With the addition of labels (nice names) we no longer listen to PHP for the default list: values/defaultReplaceVariables.js.

I propose to change this to add support for labels inside PHP so that we can keep a single source of truth.