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

locales/de.json

@@ -47,6 +47,7 @@
     "Preferences": "Einstellungen",
     "preferences_category_player": "Wiedergabeeinstellungen",
     "preferences_video_loop_label": "Immer wiederholen: ",
+    "preferences_preload_label": "Videodaten vorladen: ",
     "preferences_autoplay_label": "Automatisch abspielen: ",
     "preferences_continue_label": "Immer automatisch nächstes Video abspielen: ",
     "preferences_continue_autoplay_label": "Nächstes Video automatisch abspielen: ",

locales/en-US.json

@@ -71,6 +71,7 @@
     "Preferences": "Preferences",
     "preferences_category_player": "Player preferences",
     "preferences_video_loop_label": "Always loop: ",
+    "preferences_preload_label": "Preload video data: ",
     "preferences_autoplay_label": "Autoplay: ",
     "preferences_continue_label": "Play next by default: ",
     "preferences_continue_autoplay_label": "Autoplay next video: ",