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

src/invidious/config.cr

@@ -13,6 +13,7 @@ struct ConfigPreferences
 
   property annotations : Bool = false
   property annotations_subscribed : Bool = false
+  property preload : Bool = true
   property autoplay : Bool = false
   property captions : Array(String) = ["", "", ""]
   property comments : Array(String) = ["youtube", ""]