* Use setting.AppURL when parsing URL
Using setting.AppURL here is a more reliable way of parsing the current
URL and what other functions in this file seem to use.
* Make ComposeMetas always return a valid context
* Add per repository markdown renderers for better context
* Update for use of context metas
Now that we include the user and repo name inside context metas, update
various code and tests for this new logic
mrsdizzie