Made long running tasks cancellable. Added cancel button to cancellable tasks in the task list. Added APP_MODE env variable for determining if the app is running in development, test, or production.

cps/db.py

@@ -443,11 +443,11 @@ class CalibreDB():
         """
         self.session = None
         if self._init:
-            self.initSession(expire_on_commit)
+            self.init_session(expire_on_commit)
 
         self.instances.add(self)
 
-    def initSession(self, expire_on_commit=True):
+    def init_session(self, expire_on_commit=True):
         self.session = self.session_factory()
         self.session.expire_on_commit = expire_on_commit
         self.update_title_sort(self.config)
@@ -593,7 +593,7 @@ class CalibreDB():
                                                           autoflush=True,
                                                           bind=cls.engine))
         for inst in cls.instances:
-            inst.initSession()
+            inst.init_session()
 
         cls._init = True
         return True