Create startup blueprint for post-init logic

• take advantage of blueprint.record_once() for setup logic, which is called during registration of the entrypoint blueprints (which happens after the invenio extensions have been loaded)
• this pattern is already used in some core modules (e.g. invenio_rdm_records.views v1.0.0)

invenio_theme_tuw/ext.py

@@ -7,23 +7,11 @@
 
 """Invenio module containing the theme for TU Wien."""
 
-from flask.config import Config
 
 from . import config
 from .views import create_blueprint
 
 
-class TUWConfig(Config):
-    """Override for the Flask config that evaluates the SITE_{API,UI}_URL proxies."""
-
-    def __getitem__(self, key):
-        value = super().__getitem__(key)
-        if key in ("SITE_UI_URL", "SITE_API_URL"):
-            value = str(value)
-
-        return value
-
-
 class InvenioThemeTUW:
     """Invenio-Theme-TUW extension."""
 
@@ -35,32 +23,34 @@ class InvenioThemeTUW:
     def init_app(self, app):
         """Flask application initialization."""
         self.init_config(app)
+        self.init_blueprint(app)
         app.extensions["invenio-theme-tuw"] = self
 
-        blueprint = create_blueprint(app)
+    def init_blueprint(self, app):
+        """Initialize blueprint."""
+        self.blueprint = blueprint = create_blueprint(app)
         app.register_blueprint(blueprint)
 
+        # since invenio-app-rdm currently (october 2022) doesn't offer an easy way of
+        # overriding the provided jinja templates, we have to perform a workaround:
+        # the first blueprint that has a definition for a template (per name) gets
+        # selected by 'flask.render_template()' - thus, we just have to make sure that
+        # our blueprint is inserted before the invenio_app_rdm blueprints
+        #
+        # this has to be done after everything (including *all* blueprints)
+        # has been initialized, and is only really relevant for the UI
+        # thus, we use `app.before_first_request` rather than the startup blueprint
+        #
+        # NOTE: the below code requires Flask 2.0.2+ and Python 3.7+
+        #       for insertion-ordered dictionaries
         @app.before_first_request
-        def patch_config():
-            # replace the app's config with our own override that evaluates
-            # the LocalProxy objects set for SITE_{API,UI}_URL by casting them
-            # into strings (which is their expected type)
-            app.config = TUWConfig(app.config.root_path, app.config)
-
-        @app.before_first_request
-        def register_bp_first():
-            # since invenio-app-rdm currently (april 2021) doesn't offer an easy way of
-            # overriding the provided jinja templates, we have to perform a workaround:
-            # the first blueprint that has a definition for a template (per name) gets
-            # selected by 'render_template'
-            # thus, we just have to make sure that our blueprint is inserted before the
-            # 'invenio_app_rdm' blueprints
-            # note: the below code requires Flask 2.0.2+ and Python 3.7+
+        def register_tuw_bp_first():
             bps = {
                 name: bp
                 for name, bp in app.blueprints.items()
                 if name != "invenio_theme_tuw"
             }
+
             app.blueprints = {"invenio_theme_tuw": blueprint, **bps}
 
     def init_config(self, app):