Undo delete task operation

[jaragunde]

When a task is deleted there is no way to undo that in the UI. For that reason, delete operations are not auto-saved, so users can still refresh the browser and get the deleted tasks back. This method is a poor workaround, not intuitive and not functional for every case (there are ways to trigger an auto-save).

We should implement a proper undo operation, which would actually consist on creating a new task with the same data of the removed one. When one task is deleted, we should leave a gap where the task used to be, with an undo button there.

Task removal would be auto-saved like any other operation in the tasks screen, but users would have a chance to undo it as long as they stay in the page.

[dmtrek14]

We are in the process of rewriting the frontend for tasks. Any outstanding bugs and enhancements for the old interface will be taken into account as we implement the new design.