* feat: async content download when creating via api
Invoking the content download code in a goroutine after saving the
bookmark, this way we can return a response to the user quickly while
the webpage is donwloaded and archived.
Cache api endpont (/api/cache) remains untouched until I understand
the logic behind it.
Also updated the API endpoint for the extension, though I'm unsure why
there's a difference between the "regular" API and the webext API,
they should be using the same APIs.