Blocks: Remove is- prefix from embed alignment class

[aduth]

Related: #4118 (specifically #4118 (comment))

This pull request seeks to partially revert changes introduced in #4118, specifically with regards to the alignment class. Prior to #4118, the alignment class applied to embed blocks had intentionally targeted compatibility with common theme styling for .alignleft, .alignright, etc . These changes restore that intended class name.

Testing instructions:

Verify there are no invalid blocks in demo content (as there is on master).

Verify that aligning a block is styled correctly when viewed on the front-end. Depending on your theme, this may not have much of an impact, if any at all.

cc @Luehrsen

[jorgefilipecosta]

This looks good to me, before the align in my tests was not viewable in the frontend and now the align works (Gutenberg starter theme).
We are changing save function, so blocks created before this change will become invalid.
If the behaviour that we are partially reverting was merged before the last release, I think we should add the old save to the deprecated version so we don't see invalid block warning. If the change is recent I think it is ok to not add to the deprecated versions as the probability of someone being affected and not understanding the error is lower.

[aduth]

If the behaviour that we are partially reverting was merged before the last release, I think we should add the old save to the deprecated version so we don't see invalid block warning. If the change is recent I think it is ok to not add to the deprecated versions as the probability of someone being affected and not understanding the error is lower.

Correct, the class name change has never landed in a public release. The changes here reflect what already exists in 2.2.