Don't delete misplaced items on final attempt

In assessment mode, don't delete misplaced items from the user's state
when handling the final attempt.

This is a server-side change to match what Javascript already does: upon
submitting the final attempt, misplaced items are left on the board.
However, since the corresponding state was deleted server-side, upon
refreshing the page items were returned to the bank.

[SOL-2077] Prevent 'Change Background' button from forcing form submission

[SOL-1998] Implement Show Answer button

Miscellaneous fixes

Fix padding on the reset button so that all separators on the sidebar
end up with the same height, and add padding to the top of the sidebar
container to distance the separators from the top edge.

pylint correctly surmised that `test_grade` is an event test, so move it
over.

On the final attempt, misplaced items are not returned to the bank.
Change the "misplaced" feedback message to reflect this.

In assessment mode, when the learner has run out of attempts, enable a
"Show Answer" button.  Clicking it causes all items to move to a correct
placement.

* Consistent capitalization in the Studio editor
* Remove dangerous recommendation to delete from the top of the editor
* Stop large images from overflowing feedback popups in Studio

RTL fixes

Introduce proper right-to-left support for the following:

* Alignment of overall feedback message icons and text
* Alignment and borders of sidebar buttons
* Margin of the "You have used X attempts" message

[SOL-1805] Add "item" field to item.dropped event

Adds a new "item" field to the edx.drag_and_drop_v2.item.dropped event
data. It contains the item text label when present, otherwise the item
imageURL.

This makes it consistent with the location field, which contains the
zone text label.

Version bump: v2.0.9

Assessment mode: display item feedback

* "Negative" item feedback is displayed for misplaced items when an attempt is submitted.
* Avoid showing item-related general feedback (correctly placed-misplaced-not placed) after problem reset.
* Disabled submit button while drop_item is in progress.
* Added notes and todos about implicit invocation order some methods expect.
* Item feedback popup only close when close button is hit or another item is dragged
* Translation fixes

Prevent overlapping drop placements

* "No alignment" zone option removed
* Max items per zone setting added
* Zone and state migrations are moved to a dedicated class

Ensure old problems work on new version.

[SOL-1988] Account for decoy items in score

Recent versions of the drag and drop v2 xblock require the 'correct'
field to be present in items stored in user state, but the 'correct'
field did not exist before assessment mode has been implemented and so
problems created and completed before the introduction of assessment
mode lack the 'correct' field.

This patch adds the 'correct' field to item state at load time.
In standard mode, only items that are placed in correct zone are stored
in item state, so we can assume that any item stored in the state that
does not have the 'correct' field is 'correct'.

Move the assessment interaction tests to their own module, as the latest
addition made test_interaction.py grow beyond 1000 lines (and fail
pylint).

In order to take into account the placement of decoy items, calculate a
student's score as:

score = ((correctly placed items) + (correctly unplaced decoys)) / (total items)

[SOL-1806] Moved aria-live attributes to popup wrapper

Fix some flaky Selenium tests

Clean up studio edit template

- i18n: Use "example" instead of "e.g."
- No need to "preventDefault" for clickable elements that are now <button>s