Add unit tests for instructor tool

field containers.

previous PR (https://github.com/open-craft/problem-builder/pull/47).

The new tests check if:

- preferred block attrs are used when building block tree for current course
- block tree excludes pb-choice blocks
- blocks are correctly marked as (in)eligible
- new-style keys are handled correctly
- student_view passes calls rendering method with correct template args
- author_view and studio_view show appropriate messages

Don't cause a 500 error in the LMS if get_block unexpectedly returns None

Add an extra selector to fix CSS rule ambiguity

Fix two bugs (OC-839)

Allow toggling show_title behaviour during upgrade (OC-805)

Fix JS bug when HTML block is followed by an answer block

Visual improvements to Table block.

Instructor Tool: Drop-down for "Root block ID" section, pagination improvements (OC-783)

if "question" attribute is not set to a meaningful value.

set on block.

This ensures that export results correctly reflect changes to answers
associated with Long Answer blocks.

it works for non-eligible blocks with empty "Question title" (display_name).

InstructorToolBlockWithMixins" error that gets thrown when trying to
edit Instructor Tool blocks in Studio.

For example, ResultView should not manipulate the status area (but it
should maintain meta info about result sets).

"question" attribute.

results have been inserted into the DOM.