Entry Associations

This is the first step towards a customisable visualisation of associations.

## What This PR Does

This Pull Request creates a Drawer to visualise incoming and outgoing associations between sections and entries using the in built methods within Symphony. If the entry is in the process of being created, a message is shown.

## What This PR Does Not Do

This doesn't allow for customising the drawer contents, as this is only the first step. After this is implemented and tested thoroughly, we can address how to customise this properly.

## Thanks (like an oscar acceptance speech)

Thanks go to @brendo for his insight into this, and to @nilshoerrmann for his help with the HTML and CSS, and also for the long conversations about how we tackle this.