New navigation implementation

### This PR addresses #93, closes #95 and #96 (IMO) and contributes to #82.

# Details
The task was to simplify Projecta's navigation, both in the front-end UI and the back-end code.

### Explanation
The results are probably not as dramatic as requested. We still need to add context variables via middleware, but the use of Beautiful Soup to render the navbar is no longer required and the middleware code is easier to follow.

The broad problem is that in order to have useful links in the navbar, we need to have access to a `project` variable in order to render the links: almost all URLs require a `project_slug`. When navigating through, for example, a Committee, there is no `project` variable in the context, so we need to add the current Committee's Project to the context. 

Rather than add a `project` variable to the context in every view, it seemed more concise to use middleware to add the required variables to the context based on the variables already in the context.