Revoke outdated Blob URLs

[lindapaiste]

Partial fix for #2234

This revokes the Blob URL which is assigned to the preview HTML document. I have not addressed the additional Blob URLs which are assigned to individual .js files as that's more complicated.

Changes:

• EmbedFrame component stores its last-created Blob URL in a ref (srcRef).
• It revokes the previous URL whenever it creates a new one.
• I put the revocation inside the existing setTimeout to be safe, so that there is no chance of seeing a momentary flash of expired content.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks for working on this!!! I'm now wondering if there are any specific scenarios where folks are utilizing both the old and new blobs for their projects 😂 but this is definitely better to address!