Add feature: Timesheet / Time Tracking with Kimai/Harvest or similar

[xet7]

Originally from Felipe Silveira at original Wekan forums: https://discuss.wekan.io/t/does-wekan-offer-any-timesheet-functionality-such-as-trello-harvest/393

• track time spent on card
• total amount of hours spent by resource on each type of tasks
• create reports of useful metrics

Open Source Time Tracking, PHP/MySQL: http://www.kimai.org

Please add info if you know other Open Source time tracking. I did not find anything from Sandstorm App Market.

This is related to #43 and #62 and making Wekan programmable.

[xet7]

Related feature request from @BaobabCoder

Instead to see time spent on a card, see estimation of time that will be spent (in hours). Similar feature is in Visual Studio Team Services: https://www.visualstudio.com/en-us/docs/work/kanban/_img/filter-kb-filter-text-web.png

[xet7]

This issue was moved to wekan/wekan#812