Refactor key selection flow and allow updating keys

[gmethvin]

Adds the ability to update API keys, along with several improvements to make the user experience more consistent:

1. Factors out and standardizes common messages, command names, etc.
2. Uses the same logic from the Keys CLI in the installer
3. If the user tries to add a key that already exists, prompt them to update that existing key
4. Re-ordered the setup flow to set up an API key first before you choose the tools to install for, rather than choosing the tools, setting up the key, then configuring the tools. This makes more sense since the API key setup is completely tool-agnostic.

[andrewboni]

LGTM