put scorable first

bug fix

implements scorablexblockmixin and moves drag and drop v2 to use raw grades.

for TNL-6593

Fix typo in README.md

FIxed `test.po` -> `text.po`

Changed the order of the submit button to match capa action bar on pl…

Fix typo in README.md

Corrected "exSisting" typo

Makes draggable items more visible in IE 11

Changes the outline-style to dotted to improve visibility.

Also, IE 11 doesn't support outline-offset, which is what we use to ensure that
the draggable item outline is visible against the background.

So we've used an inset drop-shadow for normal mode.

removed duplicate msgids

a11y Interaction Improvements

See:
 https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/112#issuecomment-272210933

Implement our own SR message area instead of relying on SR tools from
edX LMS, so that screen reader messages work even when this xblock is
used outside of LMS.

It is redundant since div elements are not focusable by default anyway.

The role="group" wrapper of the problem should use aria-labelledby
referencing the problem title.

If the problem is configured to not show the title, the hardcoded
"Drag and Drop Problem" aria-label is used.

We are now using SR.readText function to prompt the screen reader to
read feedback messages, so there is no need to use aria-live, which was
sometimes conflicting with SR.readText.

The i tag is for italic and should not be used for icons.

It looks like concatenating separate feedback messages into a single
string and passing it to readText makes the screen reader read out the
feedback in its entirety more reliably than using readTexts to pass a
collection of separate feedback messages.

When the number of attempts is unlimited, we were hiding the "You have
used X out of Y attempts" text, which becomes "You have used X out of
null attempts" when attempts are unlimited.

The text was hidden, but it was still read out by the screen reader.
Do not include the text in the DOM when attempts are unlimited at all.

Wrapping each tabbed page in a <section> is an appropriate use of the
section element, but separating a single tab into multiple sections is
not.

In the past, we were using zone title as the zone ID. This is no longer
the case as we programatically generate zone IDs, so this fix is only
relevant to old problems (such as some problems from the DnDv2 demo
course).

We have already been sanitizing zone.uid to be used for zone id
attributes, but we missed zone description id attribute.

The button role was misleading and is not needed.

Usability improvements for screen reader users