Clean up external repos

- Accept full URL as input instead, which allows for non-GitHub
- Remove automatic CDN fallback in favor of adding that as an external repo if needed

(cherry picked from commit 9c899e97a97480545d022974ffd3ea1248634155)

# Conflicts:
#	app/build.gradle.kts
#	app/src/main/java/eu/kanade/presentation/more/settings/screen/SettingsBrowseScreen.kt
#	app/src/main/java/eu/kanade/tachiyomi/extension/ExtensionManager.kt
#	app/src/main/java/eu/kanade/tachiyomi/ui/main/MainActivity.kt

domain/src/main/java/tachiyomi/domain/UnsortedPreferences.kt

@@ -91,6 +91,4 @@ class UnsortedPreferences(
     )
 
     fun allowLocalSourceHiddenFolders() = preferenceStore.getBoolean("allow_local_source_hidden_folders", false)
-
-    fun extensionRepos() = preferenceStore.getStringSet("extension_repos", emptySet())
 }