Export time tracking data at the issue level

Description

We know how much time has been spent in total on an issue/MR, as well as the estimates. But we don't provide a way to easily know the evolution of time spent for this issue over time.

Description

• Add an export button on the issue/MR
• Name of the file is the issue number followed by name of the issue, with _ in place of spaces, such as 2183_export_time_tracking_data.csv
• Export the time tracking data as a csv in the following format

time spent in seconds, user #, username, full name, timestamp
3245332, 313, djaiss, Regis Freyd, 2016-10-10 12:21:54