Remove non-React templates

[DangerOnTheRanger]

While some Jinja templates are still used, many of them are made entirely obsolete by the React UI, so the unused templates should be removed. Off the top of my head, this should include:

• post-base.html
• post-view.html
• post-view-single.html
• new-post.html
• index.html
• gallery.html

There might be more, but I believe those are definitely not used by anything at the moment.