Improve the site header

[GooseNight]

This commit adds a few improvements to the site header :

• The menu items are reorganised and split into items and sub-items, the items are DIDs, RSEs, Rules and more symbolized by ...
• The switch-account feature is connected to the list items in the active accounts section
• The get-site-header feature is connected to get the list of available accounts and the currently active account and show them in the account drop down
• The logout feature is connected to the logout button
• The default UserPass login is now a form and can be submitted using the Enter key