Add alert suggesting that the user switch to a connected account. This
alert is displayed when the popup is opened over an active tab that is
connected to some account, but not the current selected account. The
user can choose to switch to a connected account, or dismiss the alert.
This alert is only shown once per account switch. So if the user
repeatedly opens the popup on a dapp without switching accounts, it'll
only be shown the first time. The alert also won't be shown if the user
has just dismissed an "Unconnected account" alert on this same dapp
and account, as that would be redundant.
The alert has a "Don't show me this again" checkbox that allows the
user to disable the alert. It can be re-enabled again on the Alerts
settings page.
An alert is now shown when the user switches from an account that is
connected to the active tab to an account that is not connected. The
alert prompts the user to dismiss the alert or connect the account
they're switching to.
The "loading" state is handled by disabling the buttons, and the error
state is handled by displaying a generic error message and disabling
the connect button.
The new reducer for this alert has been created with `createSlice` from
the Redux Toolkit. This utility is recommended by the Redux team, and
represents a new style of writing reducers that I hope we will use more
in the future (or at least something similar). `createSlice` constructs
a reducer, actions, and action creators automatically. The reducer is
constructed using their `createReducer` helper, which uses Immer to
allow directly mutating the state in the reducer but exposing these
changes as immutable.
