fix(client): Session values only last as long as the browsing session now by default.

The Session module now uses `sessionStorage` by default instead of `localStorage`. Keys such as `email`, `sessionToken`, and `sessionTokenContext` are persisted to `localStorage` as they are included in the `PERSIST_TO_LOCAL_STORAGE` array in Session.

`sessionTokenContext` was added to take the place of using the current context to decide which context was used during Sign in since it will no longer be persisted between browsing sessions. This should give us the desired behavior of the Sign out link on Settings when a user has signed in via Desktop.

Fixes #748.