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.