feat: integrated lesson markup

- added PageExtension plugin to inject custom styles scripts in a page
- removed the livecode integration and enabled PageExtension plugins for
  learn page
- also merged the profile_tab.py with plugins.py
- added a utility to find the macros from given text
- updated the before_save of lesson to find exercises using the macros
  and update the exercises as before

Issue #115

community/community/profile_tab.py

@@ -1,38 +0,0 @@
-"""
-The profile_tab module provides a pluggable way to add tabs to user
-profiles.
-
-This is achieved by specifying the profile_tabs in the hooks.
-
-    profile_tabs = [
-        'myapp.myapp.profile_tabs.SketchesTab'
-    ]
-
-When a profile page is rendered, these classes specified in the
-profile_hooks are instanciated with the user as argument and used to
-render the tabs.
-"""
-
-class ProfileTab:
-    """Base class for profile tabs.
-
-    Every subclass of ProfileTab must implement two methods:
-        - get_title()
-        - render()
-    """
-    def __init__(self, user):
-        self.user = user
-
-    def get_title(self):
-        """Returns the title of the tab.
-
-        Every subclass must implement this.
-        """
-        raise NotImplementedError()
-
-    def render(self):
-        """Renders the contents of the tab as HTML.
-
-        Every subclass must implement this.
-        """
-        raise NotImplementedError()