Individual time spent tracking information in CSV export

Continuation of exposing time tracking information in CSV export in https://gitlab.com/gitlab-org/gitlab-ee/issues/1312#note_43930253.

• When exporting issues, attach a second file only if there is relevant information in those issues. I.e. if there is no data, then don't attach a second file.
• The second file is also a csv with the following information:

Column Header	What it is
Issue ID	The issue ID of the project
Time Spent	The individual time spent recorded in seconds (per #1312 (closed))
Time Spent At	The date in which this time spent was recorded (per #1312 (closed))
Time Spent By	The user in which this time was spent (per #1312 (closed))

• No data is recorded in this csv for the time tracking entries that correspond to negative time, i.e. /spend -3h is not recorded.
• No data is recorded in this csv that correspond to the /remove_time_spent command.
• If the user enters /spend 3h 2017-10-23, an entry is created with
  • Issue ID := issue id
  • Time Spent := 3h
  • Time Spent At := 2017-10-23
  • Time Spent By := username of user who executed the command.
• If the user enters /spend 3h, an entry is created with
  • Issue ID := issue id
  • Time Spent := 3h
  • Time Spent At := Today's datestamp
  • Time Spent By := username of user that executed the command.
• Note that with this design, we are purposely ignoring when you subtract time or remove it altogether. We want to to be the simple first iteration. So it is possible that if you add up all the time spent for a given issue, it will not reflect the total time spent in the first csv file (since the number in the first csv file takes into account of subtractions of time.)
• The second file should be named similar to gitlab-org-gitlab-ee_issues_2017-10-20_times.csv