Add option to control preloading of video data

Fix #4110 by adding an option to control the preloading of video data on
page load. If disabled ("false"), the browser will not preload any video
data until the user explicitly hits the "Play" button.

If enabled ("true"), the default behavior will be used, which means the
browser decides how much of the video will be preloaded.

config/config.example.yml

@@ -718,6 +718,20 @@ default_user_preferences:
   #  Video player behavior
   # -----------------------------
 
+  ##
+  ## Automatically preload video on page load. This option controls the
+  ## value for the "preload" attribute of the HTML5 <video> element.
+  ##
+  ## If disabled, no video data will be loaded until the user explicitly
+  ## starts the video by hitting the "Play" button. If enabled, the web
+  ## browser decides how much of the video data will be preloaded.
+  ##
+  ## See: https://www.w3schools.com/tags/att_video_preload.asp
+  ##
+  ## Accepted values: true, false
+  ## Default: true
+  #preload: true
+
   ##
   ## Automatically play videos on page load.
   ##