refactor: use Flask Blueprints to organize extensions

- extensions are now blueprints: keep views, templastes and statics in the same folder
- increase app security using `flask-talisman`
- whenever possible use {{ url_for }} for links between pages
- remove references to non-existing JavaScript code
- add missing favicon.ico

lnbits/core/views.py

@@ -0,0 +1,14 @@
+from flask import render_template, send_from_directory
+from os import path
+
+from lnbits.core import core_app
+
+
+@core_app.route("/favicon.ico")
+def favicon():
+    return send_from_directory(path.join(core_app.root_path, "static"), "favicon.ico")
+
+
+@core_app.route("/")
+def home():
+    return render_template("index.html")