Add ability to disable transcodes

There could be case where someone only needs the ability to upload videos without having to transocode them.

It would also be useful while #65 is solved

wagtailvideos/views/videos.py

 from distutils.version import LooseVersion
 
 import wagtail
+from django.conf import settings
 from django.core.paginator import Paginator
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
@@ -130,7 +131,7 @@ def edit(request, video_id):
         'video': video,
         'form': form,
         'filesize': video.get_file_size(),
-        'can_transcode': ffmpeg.installed(),
+        'can_transcode': ffmpeg.installed() and not getattr(settings, 'WAGTAIL_VIDEOS_DISABLE_TRANSCODE', False),
         'transcodes': video.transcodes.all(),
         'transcode_form': VideoTranscodeAdminForm(video=video),
         'tracks_action_url': action_url,