Embed question implementation for TinyMCE

[vuvanhieu143]

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[timhunt]

@vuvanhieu143 thanks for working on this, I think we are getting close. Some comments

1. I think that for a relatively uncommon action like embedding a question, we should just have the entry in the Insert menu, but not a toolbar button.
2. Therefore, to get the grammar right, the language string should be Insert -> Embedded question (not Embed).
3. Can you use core/loadingicon to do the loading progress icon? Oh! you already were, it is just that the unused loading string was left behind.

Acutally, because this is new to me, I decided to try to fix these things myself, as a way to engage with the code more, and so understand it better. I hope that is OK.

I know the answer to 1. depends on further internal discussion, but still, I got carried away and pushed it to the main branch anyway.

Since I am new to this sort of code, please could you review the changes I made, and point out any mistakes. Thanks.