Instead of calling /sources.json and rendering the "Fetch source data"
box client-side in Javascript, call /sources.js so we can render the
html server-side.
Fixes#3807. Drops the TagScript module entirely and replaces it with
a simple call to Post.tag.
Simultaneously fixes#3773 as a consequence of removing TagScript.run.
Post.update already updates data-tags by itself, so there was no need
for TagScript.run to update data-tags as well.
