User Interactions data collection and download

## Basics

This is the only POST API endpoint at the moment. This is to allow for data collection and then download by the admin. 

* This app should be called "User Interactions".
* It will have one POST endpoint for inserting data into the database
* The admin should be able to browse the data using the usual listing views in Django
* The admin should be able to click a button and download all the data in CSV format

### POST API

The post needs to accept the following data:

* msisdn (Max 20 chars)
* action (Max 200 characters) <- REMOVE
* feature (Max 30 chars) <-- ADD
* key (Max 100 chars) <-- ADD
* value (Max 200 chars) <-- ADD
* transport (Max 5 characters)
* created_at (Auto-generated on insert)

All fields are required.