Retrieve the PAT expiration date and show it to the user in the Auth Page

[davideroffo]

Every time we are doing an HTTP request to the GitHub REST API, we should receive back a response header with the expiration date for the used token.
We would like to show the user, on the Auth Page, when the token will expire, so that the user knows when it's time to renew it.

Acceptance criteria:

• add a message to the Auth Page with the expiration date for the token used

[simoneb]

We can keep this an optional feature as users can choose the expiration when they create the token, and github notifies you when a token is about to expire. Let's prioritize other features and error handling so we can display a proper error if the token is invalid/expired

[lasmil]

Assigned to myself as it is related to:
#20

[lasmil]

After some investigation we found that we can't read the github-authentication-token-expiration header.
The reason this is happening is because this header is not exposed via CORS:

To have access to this header, it needs to appear in the headers list (as seen in the screenshot).

We will revisit this at some point in the future though.