[Tour of Beam] [Task] Fix broken Timer in ToB Run/Cancel button or hide it completely

[olehborysevych]

What needs to happen?

Timer in Tour of Beam Run/Cancel button runs forever. Need to fix it or not show it if it can't be fixed.

Issue Priority
Priority: 3

Issue Component

Component: tour-of-beam

[alexeyinkin]

.take-issue

[alexeyinkin]

Plan:

1. In PlaygroundController, remove executionTime stream.
2. Move PlaygroundController.runCode method to SnippetEditingController.
3. Add SnippetEditingController.executionStartDate, set and unset it properly when running, finishing, and cancelling.
4. Add the calculation of elapsed time to the button itself.

[nausharipov]

.take-issue

[nausharipov]

@olehborysevych @alexeyinkin is the timer needed?