Add debug mode to Rapid Diffs

What does this MR do and why?

Add debug mode to Rapid Diffs:

1. Add rapid_diffs_debug feature flag
2. Add disable_syntax_highlight request param which would disable syntax highlighting
3. Add disable_streaming request param which would send all diffs at once without streaming
4. Put rapid_diffs_disabled request param behind a rapid_diffs_debug feature flag

How to set up and validate locally

1. Enable rapid_diffs, rapid_diffs_debug and rapid_diffs_on_mr_creation feature flags
2. Go to merge request list
3. Select 'Create merge request'
4. Select branches, proceed onto next page
5. Select 'Changes' tab
6. Add &disable_syntax_highlight=true to the URL, follow it
7. Observe diffs without syntax highlighting
8. Add &disable_streaming=true to the URL, follow it
9. Observe diffs no longer streaming (streaming request should be possible to inspect with performance bar now)