Description: Adjust flask_app.py to locate template, static and theme files

On Debian, we install template, static and theme files under
/usr/share/pagure (instead of /usr/lib/python3/dist-packages/pagure,
which is upstream's default).  For that reason, we have to adjust
flask_app.py in order to make it properly locate these files.

Author: Sergio Durigan Junior <sergiodj@debian.org>
Forwarded: not-needed

--- a/pagure/flask_app.py
+++ b/pagure/flask_app.py
@@ -177,9 +177,13 @@ def create_app(config=None):
         app.register_blueprint(blueprint)
 
     themename = pagure_config.get("THEME", "default")
-    here = os.path.abspath(
-        os.path.join(os.path.dirname(os.path.abspath(__file__)))
-    )
+    # Debian installs the themes, static and template files under
+    # /usr/share/pagure.
+    here = "/usr/share/pagure"
+    if not os.path.exists(here): # build-time tests
+        here = os.path.abspath(
+            os.path.join(os.path.dirname(os.path.abspath(__file__)))
+        )
     themeblueprint = flask.Blueprint(
         "theme",
         __name__,
@@ -188,6 +192,8 @@ def create_app(config=None):
     )
     # Jinja can be told to look for templates in different folders
     # That's what we do here
+    # Debian install templates under /usr/share/pagure.
+    app.template_folder = os.path.join(here, "templates")
     template_folders = os.path.join(
         app.root_path,
         app.template_folder,