feature/lti_config_overrides

This PR adds the ability to selectively override LTI tool config XML values by setting GET parameters. 

In particular, this allows the developer to override the button text that appears in the Canvas course navigation so that tool instances from different environments (localhost, sites.dev, flashcards.fas, etc) can be easily distinguished.

For example, to change the course navigation text from *Harvard Cards* to *Harvard Cards (localhost)*, set the following GET parameter on the tool config:

`http://localhost:8000/lti_tool_config?course_nav_text=Harvard%20Cards%20(localhost)`

As a result, the XML config should produce *Harvard Cards (localhost)* as expected. Otherwise, if no parameter is set, then it should default to *Harvard Cards*.

@jazahn Can you review this?

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[FLASH-154](https://jira.huit.harvard.edu/browse/FLASH-154)