Add persistence to API keys

[rzats]

Closes #63.

• Introduces a new store named apiKeySelections. This is populated when the application is initially launched, and saved in the browser's local storage.
• This stores API and auth keys across different signals.
• Also adds a checkbox to prevent API keys from being saved, which cleans the store immediately when deactivated.

[melange396]

Can we change this so that all of the api key and authorization token fields across the different endpoints share the same singular backing store? That will save the user from having to enter their key into the forms of each individual endpoint, plus our permissions model allows us to grant permission to multiple endpoints via just one key (so a user should never need a plurality of keys).

Can we move the "Save API keys & auth tokens" checkbox next to or right above the "FETCH DATA" button, and can we make it default to unchecked?