Rescue merging with invalid custom config

If custom config contains invalid indentation merging with Hound's default
config will raise an exception:

https://app.getsentry.com/app20126465herokucom/production/group/29978234
Add account page

* show repo totals
* show a list of billed repos
* refactor external customer data fetching to `PaymentGatewayCustomer`

https://trello.com/c/6AyMCv1t