Allow event manager to manage tasks during event creation

[clain-atomi]

Design - https://www.figma.com/file/YTqnsudolN9Duj00VezOcb/Gandalf_Design?node-id=515%3A833

1. When user click to + button new empty task is created

2. Task header allows task name update, if name is too long it truncates using ellipses.

3. Show empty hint if there are no tasks

4. Support 4 types of tasks (single answer, multiple answers, coding task, text answer)

5. For each task type user see "mentor check" checkbox which represents do we require this task to be reviewed by mentor. Max score field (only positive numbers allowed). And task description with ck4editor (already implemented, just reuse)

6. For coding task user see additional "unit tests area' with code editor to choose (both already implemented, just reuse) to write unit tests for tasks. Mentor check can be enabled or disabled (disabled by default).

7. For single answer task user additionally can manage answer options and mark one answer as truthy by selecting it as radio button. Mentor check is disabled and false by default.

8. For multiple answers task user additionally can manage answer options and mark any of them as truthy by selecting their corresponding checkboxes. Mentor check is disabled and false by default.

9. Ignore drag and drop for tasks for now

10. For text task type user mentor check is true and disabled.