Fix broken side-by-side diff view on merge request page

### What does this MR do?

This MR fixes the side-by-side diff that was broken in v7.9.0. The "Inline" and "Side-by-Side" buttons are now always rendered with HTML links instead of changing depending on whether the code diff was requested in JSON format.

### Are there points in the code the reviewer needs to double check?

Is there a better way to fix this? The links in JSON format may be desirable if there were a JavaScript hook to handle the buttons, but as far as I can tell there isn't at the moment.

### Why was this MR needed?

The issue is that the code diffs can be generated either by the JavaScript handler or the regular HTML. With the forcing of JSON request format in 27e75344, the JavaScript handler requests the page in JSON format, which it then uses to render the page. However, this has the unintended side effect of requesting that the "Inline" and "Side-by-Side" buttons have references to a JSON-formatted link.

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

#1294

See merge request !460