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/routes/preferences.cr

@@ -27,6 +27,10 @@ module Invidious::Routes::PreferencesRoute
     annotations_subscribed ||= "off"
     annotations_subscribed = annotations_subscribed == "on"
 
+    preload = env.params.body["preload"]?.try &.as(String)
+    preload ||= "off"
+    preload = preload == "on"
+
     autoplay = env.params.body["autoplay"]?.try &.as(String)
     autoplay ||= "off"
     autoplay = autoplay == "on"
@@ -144,6 +148,7 @@ module Invidious::Routes::PreferencesRoute
     preferences = Preferences.from_json({
       annotations:                 annotations,
       annotations_subscribed:      annotations_subscribed,
+      preload:                     preload,
       autoplay:                    autoplay,
       captions:                    captions,
       comments:                    comments,