Add iOS/Android nightly builds + release build publishing

[kkaefer]

After #10257 landed and we disabled all Bitrise builds for master, we can now start working on automatically building nightly releases for iOS/Android, as well as automating release publishing when pushing tags.

/cc @tobrun @friedbunny

[tobrun]

An example of reacting tag creation can be found in the demo app repo (there we release a new version of the application to Google Play when a new tag is created).

[friedbunny]

Per CircleCI docs, it seems that if you want to make a job tag-only, you must also explicitly ignore all branches:

- ios-release:
    filters:
      tags:
        only: /ios-.*/
      branches:
        ignore: /.*/

[friedbunny]

iOS release builds happen on CircleCI as of #11437, but nightly builds and metrics haven’t yet been migrated from Bitrise (and aren’t currently running at all).

[tobrun]

Both iOS as Android are using a tag based system. Closing.