Add maxlength attribute to authoring editor textareas

[ORA-735](https://openedx.atlassian.net/browse/ORA-735): textarea tags should have maxlength attributes.

I chose 10,000 characters arbitrarily, except for:

1) AI examples, which I didn't set a limit for because it may need to be extremely large and won't be visible to most course teams.
2) Student training examples, which use the same size limit as student responses (100,000 chars)

@stephensanchez please review.