created show notes for frozen videos
[Finishes #82528944] Add ability to embed Blockly blocks within multi/match content
cleanup based on feedback
Merge pull request #572 from code-dot-org/staging

DTT
Merge pull request #575 from code-dot-org/test

DTP
Merge pull request #577 from code-dot-org/staging

DTT
do not allow changing only case when editing level name
better error message
Merge branch 'staging' into test
Upgraded /images in Pegasus to better support Retina assets.

Moving forward, ideally the *only* images that would be checked in would be the retina `@2x` versions, e.g. `/images/bearclaw@2x.png`. Requesting `/images/bearclaw.png` will resolve the `@2x` version and reduce it to half the original size when delivered. If both `bearclaw@2x.png` and `bearclaw.png` files exist, the right file is selected an returned unmodified.

Now, **all** images support automatic type translation, e.g. when `/images/bearclaw.jpg` is requested Pegasus will locate `bearclaw.png` (or `bearclaw@2x.png`...) and translate the image appropriately.

The manipulation features (`fit`, `fill`, `resize`) now support height-only specifications in addition to width-height, and width-only ones. For example, `/images/fit-x100/bearclaw.jpg` will constrain only the height when resizing.

PNG is the preferred format for images when committed, but JPG is almost always the preferred format when referencing on the page. A 300 KB PNG is generally <60KB as a JPG reducing our bandwidth costs and improving site performance. The most frequent exception is when an image contains transparency that needs to be preserved.
Shift learn tabs slightly to the right.
Merge branch 'staging' into test
Merge pull request #579 from code-dot-org/test

DTP
add cond blocks to eval and play lab.
hotfix educator notes link
Merge branch 'test' into staging
check language for i18 and mobile carousel
educator notes and check language for learn_carousels
Dropbox changes.
Merge pull request #580 from code-dot-org/test

DTP