Make custom domain name optional

[lmammino]

Right now it is necessary to have a custom domain which makes the first setup unnecessarily complicated and it makes it hard for people to just try to install weshare.

It might be good to figure out how to use AWS-generated domain names (through API Gateway URLs) by default.

This requires a bit of research and there might be some significant pain points:

• We currently assume there's only 1 base URL and we rely on that to build the other URLs (${base}/auth and ${base}/share). If we cannot rely on a custom domain we would need to either merge the auth and the backend stack to have the same API Gateway or figure out a way to support multiple base URLs. Similarly, the prefixes /auth and /share are currently the result of using custom domain mappings with API Gateway. We would need to either recreate them as actual paths under the API gateways or make the CLI smarter to use these prefixes or not depending on whether a custom domain is being used or not.
• Doing conditional resources in CloudFormation is notoriously painful...

[eoinsha]

This example of conditional domain resources in CloudFormation might help ease the pain: https://github.com/fourTheorem/slic-starter/blob/25ac05419f98af2ce7f0c5de9f89e0981a5b1aba/packages/checklist-service/serverless.yml#L128