Add release channel support to differentiate environments

[teddyviking]

This PR adds support of release channel to be able to set different environments in CI

Reference: https://docs.expo.io/versions/latest/guides/release-channels.html

[jevakallio]

@teddyviking thank you for this! I haven't used Expo release channels so far. Do you think it would be feasible to replace the home-grown namespacing appr currently does by modifying the name of the app entirely by just creating a new release channel for each branch?

Reading the example workflow documentation, it doesn't sound like there would be a problem creating transient channels:

If you have a new version that you dont want v1 users getting, release v2 of your app by running exp publish --release-channel prod-v2 and building it with exp build:ios --release-channel prod-v2. Users with the prod-v2 ipa will only be pulling releases from that channel.

This way we could stop polluting Expo with orphaned private apps.

[jevakallio]

@teddyviking I seemed to have accidentally closed this PR - apologies. Reopened. May not land this exactly as-is, we are looking to migrate to release channels as the primary way of deploying PRs.

[jevakallio]

@teddyviking thanks for this PR! I couldn't merge this diff since we decided to go a bit different direction and use release channels as the default publishing strategy from now on.

This feature has now landed in appr@2.0.0:
https://github.com/FormidableLabs/appr/releases/tag/v2.0.0