Add-ons have a restricted scope for onOpen, so I made the CacheLoader a local variable within two functions
Narrowed scope down to only three scopes necessary for the add-on to function. UrlFetchApp doesn't require permission when it runs within a custom function, but it does when it's excuted in the context of the sidebar, so I added it later.
The very simple sidebar with card system should be tested and then expanded with instructions. For now, most important question is: does sidebar work and does it register user's email to hubspot?
Some small fixes