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

The button would be too close to the input field on small screens.

- Change a elements to buttons.
- Don't use placedholers.
- Make font sizes more consistent.
- Fix color contrasts.
- Use location.html_id() built-in function.

ID attributes have to be unique on the document level. To avoid
accidentaly having multiple elements with the same ID, we prefer to use
class attributes.

There are two situations where we still need an ID attribute:

- Connecting <label> elements to the associated input element using the
  'for' attribute
- Connecting description of an element with the 'aria-describedby'
  attribute.

In the first case, we can often wrap the associated input inside the
<label> tag, so that we don't need IDs, although in some cases that is
not possible because it breaks the layout.

In cases when we still need to use ID attributes, we append a random
string to ensure uniqueness.

- Use field.display_name instead of hardcoding field names in the
  template.
- Display help text under each field instead of putting it in the title
  attribute.

Version bump: v2.0.8

Click handlers and backend processing for assessment mode

README sections about localization

[SOL-1944] Submit Answer UI

[#SOL-1944] Review notes: responsiveness, ARIA attributes, focus after reset

Revert "Add some documentation of assessment mode to README.md."