[ios, build] Add tag-based Bitrise iOS deployment workflow

[friedbunny]

Closes #2844 by using Bitrise to build and deploy iOS releases.

The release process will be as described in #2844 (comment) — the new step is 3.:

1. Release manager bumps versions and updates release notes; gets that merged.
2. Release manager tags the release.
3. Bitrise receives the tag hook and starts the release build using the deployment script.
4. Release manager updates the Github release with notes, checks that binaries are 👍, then pushes publish.
5. Release manager handles the various docs updates elsewhere.

Notes

• It takes about 50 minutes for Bitrise to setup, build, and deploy — versus 30-40 minutes locally.
• Using make ideploy locally will still work, provided you do all of the usual auth’ing.

/cc @fabian-guerra @boundsj @1ec5 @jmkiley

[friedbunny]

Updated the release instructions in the wiki.

[boundsj]

This is great! Thanks @friedbunny