* Rename Upload#download_for_upload to #get_file_for_upload.
* Fix #get_file_for_upload to raise error if no file or source url was given.
* Fix javascript upload validation to disallow submitting form if file is
not present and the source is not an url.
Initialize the recent tags, frequent tags, translated tags, and artist
tags columns only once, when the related tags section is first shown.
The related tags section is shown automatically when the 'Edit' tab is
opened, or by default on the uploads page.
This fixes the /related_tags.js and /source.js calls being triggered
every time the tag edit dialog box or the 'Edit' tab was opened.
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.
