Step navigation and submission for new mentoring block

[itsjeyd]

This PR adds step navigation and submission functionality to the new blocks introduced in #62.

• New mentoring block now displays one step at a time.
• New step block allows to submit step results. Info about last step submitted is saved at mentoring level so users are taken to the correct step when resuming their work (or reloading the page).
• New mentoring block allows to navigate to next step after submitting step results.
• After submitting the last step, new mentoring block displays a "Try again" button that allows to reset the block and start from scratch. (This functionality will be moved to the review step as part of OC-913.)

Screenshots

New mentoring block displaying a single step (with multiple questions):

After submitting the current step (all answers correct):

After submitting the current step (some answers correct):

After submitting the current step (no correct answers):

[e-kolpakov]

@itsjeyd Is there any particular benefit of not using Dict field?

[itsjeyd]

@e-kolpakov Hm. Maybe for the odd case where a question (with a non-unique name) is used multiple times within the same step? With a List field we'd be able to retain results for all occurrences of the question in that case.

[e-kolpakov]

@itsjeyd this is unlikely to work at all - how do you than distinguish the questions when you need to display their values? By sequential number? I'm not sure it would work (but not sure it wouldn't as well). Let's keep it as is for now, but please keep in mind it might make sense to use Dict.

[e-kolpakov]

@itsjeyd there's one minor issue in studio editing - not sure if it slipped from the previous task or was added as part of this one:

"Que?" there is a "Question" field of an MCQ - looks like student view is reused for preview view. If it's not clear, the screehnshot is made when editing MCQ answers.

[itsjeyd]

@e-kolpakov Thanks for the comments. I looked into the studio editing issue -- it's present in the existing mentoring block as well and it seems like it's caused by a peculiarity of MCQBlock: Unlike other types of question blocks it adds the value of self.question to its Fragment in get_author_edit_view_fragment:

def get_author_edit_view_fragment(self, context):
    """
    The options for the 1-5 values of the Likert scale aren't child blocks but we want to
    show them in the author edit view, for clarity.
    """
    fragment = Fragment(u"<p>{}</p>".format(self.question))
    self.render_children(context, fragment, can_reorder=True, can_add=False)
    return fragment

This behavior was introduced way back in Feb -- let me know if you think I should still change it as part of this PR.

[e-kolpakov]

@itsjeyd if it's an existing behavior - let's just keep it. I just haven't saw it before for some reason, so I thought it was a regression

[e-kolpakov]

@itsjeyd 👍