sessionReducers: Refactor APP_REFRESH handler to match others.
sessionReducers: Refactor APP_REFRESH handler to match others.

Combine the code responding to `APP_REFRESH`, `ACCOUNT_SWITCH` and
`LOGIN_SUCCESS` as it is the same (sets `needsInitialFetch` to `true`).

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile