Preferences: Add option to control preloading of video data (#4122)

This PR adds a configuration option to control the preloading of video data on
page load with the HTML5 'preload'[1] attribute on the `<video>` element.

The option is enabled by default, meaning that the `preload` attribute's value
will be 'auto'. If users want to prevent preloading of video data, they
can disable the option, which will set the attribute value to 'none'.

[1](https://www.w3schools.com/tags/att_video_preload.asp)

Closes issue 4110

config/config.example.yml

@@ -707,6 +707,22 @@ default_user_preferences:
   #  Video player behavior
   # -----------------------------
 
+  ##
+  ## This option controls the value of the HTML5 <video> element's
+  ## "preload" attribute.
+  ##
+  ## If set to 'false', no video data will be loaded until the user
+  ## explicitly starts the video by clicking the "Play" button.
+  ## If set to 'true', the web browser will buffer some video data
+  ## while the page is loading.
+  ##
+  ## See: https://www.w3schools.com/tags/att_video_preload.asp
+  ##
+  ## Accepted values: true, false
+  ## Default: true
+  ##
+  #preload: true
+
   ##
   ## Automatically play videos on page load.
   ##