Add a read replica configuration for the database

### User Story

As a Gitcoin site maintainer, I want a read replica configured for the database, so that database queries are faster and we can scale database load horizontally.

### Why Is this Needed

[comment]: # (Describe the problem and why this task is needed. Provide description of the current state, what you would like to happen, and what actually happen)
Grants round 8 was the most successful grants round by far, and resulted in the highest server loads we've seen. As many of our database queries and server endpoints have complex business logic (which likely can be simplified, but beyond the scope of this ticket) being able to scale the database will help us support higher server loads and keep the site performant.

### Description

[comment]: # (Feature or Bug? i.e Type: Bug)
*Type*: Feature

### Current Behavior
The django codebase only supports one database

### Expected Behavior
[comment]: # (Describe what you expected to happen.)
The django codebase supports one write database, with read replicas

### Definition of Done
The additional configuration to support read replicas is added to the django configuration

### Data Requirements

Site performance will improve and the upper limit on site activity will be raised