Add documentation for manually deploying the UI in an AWS account

[savingoyal]

Besides the bundled CloudFormation template, add documentation on how the UI can be deployed (including logical replication of the DB) using the AWS console.

[saikonen]

first draft ready for review at Netflix/metaflow-docs#14

had some open questions on how much detail we should go into with the guide, so I followed the metadata-service guide style for now.