Upgrade to using `potions` underneath `galah_config`

[mjwestgate]

galah_config() requires an upgrade for the following reasons:

• New APIs require different information to be stored for each atlas (#163)
• Methods for using confidential information are not well documented
• Code for storing and retrieving data is messy (e.g. getOption("galah-config")$package$verbose)
• Current solution has some bugs (e.g. #178)

Proposal to test using potions (https://potions.ala.org.au) to resolve this.