refactor(frontend): settings and friends

[amanape]

Summary

This PR tests, rebuilds, and refactors code related to anything Settings so that it will be easier to work with in future updates. It also introduces a new feature of disabling modal actions.

Motivation

Extending settings proved to be a more annoying task than it should be (#1319) because of its current state. Responsibilities were mixed between functions, and some were outdated (settings Redux store).

Main Changes

• Move API handlers to dedicated folder
• Move initializeAgent to dedicated service folder
  • Modify function definition (initializeAgent(settings: Settings): void)
• Rebuild settings API
• Extend <BaseModal/> to support disabling actions
• Retire Redux settings slice

Improvements

• Everything modified has complementary tests (TDD except for initializeAgent)
• Settings type is no longer a generic record type
• Disable save button if no settings have been changed or if empty value
• Separation of concerns/responsibilities
• JSDoc on new/updated service functions
• Settings is easier to read/extend

[rbren]

This looks much cleaner! I'll wait for some FE folks to look but LGTM overall