Fix pasting and cmd insertion into the SupSub block of a MathFunction.

[drgrice1]

The SubSub block is supposed to only allow a SubSub (i.e., a subscript or superscript) to be entered. Anything else should move to the function parameter block and be inserted there. However, if something is pasted from the clipboard or the API cmd method is used to insert a latex command (as is done by the PG mqeditor toolbar), then that is being allowed in the SupSub block. So this fixes those cases to only allow SupSubs as should be the case. Anything else gets moves into the parameter block.

Note that this is built on top of #42. Also, this has not yet been published. You can test this with webwork2 and PG by cloning this pull request branch, running npm install, npm run build, and npm link, and then in the pg/htdocs directory running npm link @openwebwork/mathquill. Note that to test the cmd case with the mqeditor toolbar you will need to do this.

I will publish this to npm once merged.

[pstaabp]

I think I have it set up correctly, but am trying to test this. Can you clarify what was not working or how to test. Is this nesting of button templates?

[drgrice1]

Type log in the MathQuill input. After typing that the cursor will be blinking before the parenthesis in the "SupSub" block where you are only supposed to be able to type a superscript or a subscript. Then paste something there. It will be inserted there with the main branch and should not be. With this pull request it will go inside the parentheses where it should go.