Templates: Always parse blocks from content

[aduth]

Fixes #9433

This pull request seeks to resolve an issue where no demo content is shown on the demo screen. It does without sacrificing the original intent of #9288, which had been the cause for the regression described by #9433.

Post content is always parsed, regardless of whether a template is assigned or the post is new or being edited. This allows the initial demo content to be parsed as blocks.

If a template exists and the post is new, only then is the template applied by synchronizing. In most cases, the initial content would have been empty and the blocks an empty array (as had existed in previous logic), but this also accounts for potential divergences between content of the new post and the template.

Testing instructions:

Repeat steps to reproduce from #9433, verifying demo content is shown.

Repeat testing instructions from #9288.

Ensure end-to-end tests pass:

npm run test-e2e

[youknowriad]

Seems legit code-wise. Can't test at the moment though. Feel free to merge if you're confident otherwise I'll test tomorrow.

[youknowriad]

Tested and confirmed that it works great.