Refactor tcms.testcases.views.plan_from_request_or_none method

[asankov]

Currently, this method accepts both GET and POST request. We need to examine carefully whether this is the desired functionality.

Also, the current implementation returns either TestPlan object, an id or None. In my opinion, returning just the id is useless, because if you need just the id you can get it directly from the request, without calling this method. Moreover, this functionality (that returns just the id) is used at only one place. This means we can easily remove it from the method.

[asankov]

Currently, this method accepts both GET and POST request. We need to examine carefully whether this is the desired functionality.

This method is called 6 times. At least 1 is GET and at least 1 is POST, so this behavior must be kept for now.

Also, the current implementation returns either TestPlan object, an id or None.

This has been refactored and now it is now returning either None or a TestPlan object.

I am closing this, because I think this method cannot be simplified further.