Fix escaping issues with HTML entities

[donnapep]

Fixes #152.

This was tricky as the browser's HTML parser kept converting HTML entities like " to ". Fortunately, WordPress already solved this problem in their Code block, so this solution borrows from that.

Testing Instructions

1. Add a SyntaxHighlighter Code block to a post using the version from master. Content to use is given in SyntaxHighlighter Code block: improper encoding/decoding of HTML entities #152.
2. Switch to this branch and add a SyntaxHighlighter Code block to a new post using the same content as above.
3. Ensure both the editor and the front-end render properly.
4. Switch to the old version and ensure it still works.

[alexsanford]

This seems to work well for newly created posts 🎉

I'm not sure what this is supposed to do in terms of migrating old versions of this block. Is it intended to fix old versions?

I'm doing the following:

• Move to master.
• npm install && npm run build
• Create a post, add the block, paste in the content, save.
• Load the frontend (see some characters not displaying properly).
• Switch to this branch.
• npm install && npm run build
• Reload the frontend.

At this point, I see this:

And this:

And this:

Two things:

• The entities don't seem to be fixed in migration. Is that something that we should expect?
• The <pre> tags have been visibly added. They aren't removed until I open the block editor and save the post.

[donnapep]

I don't think this is going to be fixable for existing blocks, because the solution requires a change to the markup (to add a <code> element), which you can't do because the editor detects a mismatch between existing post content and the save function. Hence the deprecation.

[alexsanford]

Maybe I don't understand what the deprecation is supposed to do. I don't think I got a notice or anything when I upgraded, it just quietly changed the output of the block. I'm concerned that this will happen for our users too.

[yscik]

[deprecation] just quietly changed the output

That's the point, it's basically a compatibility tool to read block data from old versions, and use it seamlessly.

I think there might be a way to fix the entities in old versions in the deprecation's migrate function, but it would be so overly complex that it's probably not worth pursuing.

[yscik]

Fix works well, code looks good