Fixes #3380 - Makes Wizard Form the default (python)

[karlcow]

This PR fixes issue #3380

Proposed PR background

(env) ~/code/webcompat.com-karlcow % git log 23f2665a..73dcff1f                                
commit 73dcff1fb95acf013328de6461a4ac911070c9ce (HEAD -> 3380/1, origin/3380/1)
Author: Karl Dubost <karl+github@la-grange.net>
Date:   Tue Jul 14 11:37:29 2020 +0900

    Issue #3380 - Makes WizardForm the default form
    
    So this is doing a couple of things.
    
    - It removes the duplicate problem_choices list of 2 items tuples.
    - it removes the IssueForm class
    - it transfers the IssueForm attributes into FormWizard
    - it reorders in some broad categories the fields  for the form
    - it switches getform to FormWizard. (it was not the case!)
    - it adjusts the tests

commit 304d11a7c23ac845ef192cb3d00671ec81ee888b
Author: Karl Dubost <karl+github@la-grange.net>
Date:   Tue Jul 14 11:32:58 2020 +0900

    Issue #3380 - accepts list with 3 and 2 items tuples
    
    Originally the list of tuples were 2 items long.
    but the wizard is using in some cases 3 items long list (because of icons) for radio fields.
    
    This leds to the duplication of the list of problem_choices with 2 items and 3 items. So instead of duplicating code, let's handle it in the function itself. There might be something more elegant to do. We can discover that later.

commit 09853fb7abf48f77866840c70d4a3a928b53ae7c
Author: Karl Dubost <karl+github@la-grange.net>
Date:   Tue Jul 14 11:28:52 2020 +0900

    Issue #3380 - simplifies PrefixedRadioField
    
    This optimizes a bit the code to make it easier to read. It doesn't change the global logic

commit 23f2665af5333ade44833f9c9a71b115f2d9ed52
Author: Karl Dubost <karl+github@la-grange.net>
Date:   Tue Jul 14 11:25:59 2020 +0900

    issue #3380 - Cleans up code
    
    These are breadcrumbs of dead code and useless redirection of variables.
    
    Let's remove them before modifying the Form.

[miketaylr]

Thanks Karl.

[miketaylr]

(we should be sure to test carefully on staging here, just to make sure we don't break reporting :))

[karlcow]

(we should be sure to test carefully on staging here, just to make sure we don't break reporting :))

Definitely.