Create user context header.

The creation of a new context-dependent header system to handle each of the following contexts:

• Site
• Category
• Course
• User

Each header should contain at a bare minimum, a title and breadcrumbs, with a design that provides "attachment points"...? (wording to be determined) for additional functionality later (e.g. a "message this user" link when viewing another user's user-context page).

Layout should be consistent throughout the different contexts to reinforce user familiarity.

Specification for this task https://docs.moodle.org/dev/Navigation_overhaul_specification#User_context
See the invision web app for mockup of these pages. http://invis.io/WY1V3HF9D

Steps for completing this issue:

1. Defining a context header renderable that will contain
   1. Image
   2. Title
   3. Subtitle
   4. Actions (action menu renderable or so)
2. Defining a header and subheader renderable containing:
   1. A context header renderable.
   2. A breadcrumb for the "header" renderable.
3. Defining a breadcrumb renderer and renderable if they do not currently exist.
4. Using the above to create a user header and subheader.