Self-host support

[vin047]

• Refactor code to support custom S3 endpoint.
• Refactor code to support custom server endpoint.
• Refactor API caller to support 2 endpoints – one for self-hosted server and one for central server calls (for calls related to Firebase service and notifications).
• Design user interface to support custom endpoint entry on user sign in.
• Call API to set token claim and refresh token once custom endpoint is entered.
• Force refresh token when creating a new user.
• Force refresh token when authorisation details are updated/changed.
• Update README.