[#797] Insert display name when generating RAW DATA report

While generating the raw data report, look at questions selected to appear as *Display name* and insert their values into the respective column in the report.
Just a _style_ change. This could be rewritten as:
````java
  for (String displayName: displayNameQuestionIds) {
  // do something
  }
````
So you don't need to keep track of indices etc.

We had the same conversation with @ichinaski and this loop is doing "too much". On each loop we verify if we're last element. IMHO, It's "simpler" to just remove to trailing ` - ` at the end after building the string.