make SubtaskTimeTrackingExportCommand work and export to CSV, not html

[liedekef]

This commit make the SubtaskTimeTrackingExportCommand work (next to AllSubtaskTimeTrackingExportCommand) and exports to CSV as the rest of kanban does (this could result in some html-generating code to be removed).
Next to that a css part is removed that interferes with the latest kanboard code to allow timespent to be entered directly on the task upon creation too.

[dlazesz]

Dear @liedekef,

In Plugin.php at line 8 the definition of SubtaskTimeTrackingExportCommand is missing, so an exception occurs.

To address this problem, please consider adding the following line after line 8:

use Kanboard\Plugin\Timetrackingeditor\Console\SubtaskTimeTrackingExportCommand;

Otherwise, I personally really like this addition and hope it to be merged soon!

[liedekef]

Indeed, I fixed that already in my version, but forgot to add the use-line to the git source.

[dlazesz]

Thank you!
Regarding the exported CSV, I would prefer to see not just the days, but also the time for the starting and ending timestamp.
Do you agree with changing SubtaskTimeTrackingExport.php#L125-126 from date() to datetime() or it is just me?

[liedekef]

I'll take a look and see if that works and if so, change it.

[liedekef]

Ok, since the command export:allsubtaskstimetracking already exports to datetime, I've changed that now for export:subtaskstimetracking too.