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/__init__.py

@@ -0,0 +1,8 @@
+from flask import Blueprint
+
+
+core_app = Blueprint("core", __name__, template_folder="templates")
+
+
+from .views_api import *  # noqa
+from .views import *  # noqa