Resolved T3382 - Oh noes guy flashing

Summary:
Although we cancelled the timeout to show oh-noes-guy when hide() was called,
timeouts could hang around if they were superseded by another call to toggleErrors()
without calling hide in between. These stragglers would fire later on and create the
effect of oh-noes0guy flickering

Test Plan:
- Watch the talkie mentioned in the task
- Don't see flickering

Reviewers: pamela, john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14676
Remove collies, redundant with dog-collies

Test Plan:
- Insert image in webpage, dont see collies in animals photos tab
Add target="_blank" to all links

We had an issue where clicking on a link did actually
attempt to replace the frame, due to the link having same target
and the browser executing that before the onlick seemingly,
and that resulted in DOM security errors that persisted in playback.
This adds target=_blank to all links so that security error
does not happen.
(We could also handle security errors better if we run into
others in the future, to not break experience as badly)

Test Plan:
- Watch https://www.khanacademy.org/computing/computer-programming/html-css/html-tags-continued/p/html-links
 until the point where the link is inserted. Pause and click the link.
- Continue playing- you should *not* see an Oh Noes error.

Auditors: brian

Subscribers: alexrodrigues, #cs
Add the ability to prevent pasting in content to the Ace editor, brought over from KA.org.
New custom constraint and a callback fix

Summary:
Added a new custom constraint for isValidColor, now that custom constraints work.
I'll use it in a few challenges (I don't actually use notDefaultColor, since we don't
actually do a default color, so I could just delete that now).

I also reworded the Oh Noes messages to be more in the PJS style.
I was getting confused by their wording myself, as to whether they were telling me
what *should* be the case or what *was* the case.
I imagine I'll keep iterating as I see more challenge feedback come in.

Test Plan:
- Run live-editor tests
- Make a few errors while writing HTML, see the new messages

Reviewers: alexrodrigues, john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14701
A number of bug fixes from structured-blocks.
Merge branch 'master' of github.com:Khan/live-editor