[Server support] - Add Repository that abstracts authentication

[matuella]

As the server is introduced, we need to allow users to authenticate, so we can store each individual progress in Firestore.

Before implementing this, we need to have:

• [x] Decide if the application can be used anonymously while the user doesn't sign in. Answer: it won't allow anonymous users, due to not have to deal with local/cloud sync stuff - we can eventually support it, but not right now.
• [x] Authentication UI.

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Sub-issue that relates to the Server support - https://github.com/olmps/memo/issues/136.