[Web] Handle expire session/browser reload when editing identity

[tddang-linagora]

Description

When user are creating or editing signature, session might expire or user choose to reload the browser. Twake Mail should warn the user of their action, and restore the signature editor with the content intact.

DoDs

• Show warning dialog when user reload browser or when user's action trigger back-end request and their session is expired
• Cache signature content before reload, then restore after page is loaded.