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

app/src/main/java/eu/kanade/tachiyomi/extension/ExtensionManager.kt

@@ -3,7 +3,7 @@ package eu.kanade.tachiyomi.extension
 import android.content.Context
 import android.graphics.drawable.Drawable
 import eu.kanade.domain.source.service.SourcePreferences
-import eu.kanade.tachiyomi.extension.api.ExtensionGithubApi
+import eu.kanade.tachiyomi.extension.api.ExtensionApi
 import eu.kanade.tachiyomi.extension.api.ExtensionUpdateNotifier
 import eu.kanade.tachiyomi.extension.model.Extension
 import eu.kanade.tachiyomi.extension.model.InstallStep
@@ -49,7 +49,7 @@ class ExtensionManager(
     /**
      * API where all the available extensions can be found.
      */
-    private val api = ExtensionGithubApi()
+    private val api = ExtensionApi()
 
     /**
      * The installer which installs, updates and uninstalls the extensions.