Add CORS support

[abatkin]

Add CORS support to linkding. This is useful if you are running an alternate web-based frontend. By default, all CORS support is disabled, but once one of the documented environment variables are set, an additional Django middleware (django-cors-headers) is added and configured appropriately.

This change:

• adds a new dependency django-cors-headers (which adds no additional net-new transitive dependencies)
• adds a number of new configuration parameters/environment variables
• documents the settings on the Options page

[garyp]

I'm working on an alternative web-based frontend and also had a need for the Linkding API to return CORS headers. I just tested this PR and it works great (at least for my use case).

[abatkin]

Rebased against master