Rename catarse_fee configuration to platform_fee

Despite Neighborly being a fork of Catarse, there is no need to associate the source with with Catarse.

With the deploy of this branch, we will need to run the following command in server's console:

```ruby
catarse_fee_config           = Configuration.find_by(name: 'catarse_fee')
Configuration[:platform_fee] = catarse_fee_config.value
catarse_fee_config.destroy
```