[UX] Maintenance mode: Do not render the "Create new account" tab, the navigation menu and the account menu when in the "Log in" or "Reset password" pages.

This is how the "Login" and "Reset password" pages are rendered when in maintenance mode (for non-authenticated users):

What's wrong here:

• the "Create new account" tab leads to the maintenance page, so it should better not be rendered at all in the first place
• it's simply silly to be rendering the account menu ("Login" link) when the user is already in the login page. If they are in the password reset page, they can simply use the "Log in" tab. Even if for some "exotic" reason the site theme does not render the primary tabs, it still would not matter because the site is in maintenance mode, which means that the user either followed a link to /user/password or they are advanced users and typed the URL themselves.
• the navigation menu (any menu actually) should not be rendered for similar silliness reasons: the site logo/name serves the purpose of navigating to the site's home page, so (assuming that the default "Home" link has not been removed/disabled from the main navigation menu) there is no critical navigational role that this menu plays when in maintenance mode ...especially since the rest of the links in it won't work (they all redirect to the maintenance page)
• There is no indication that the site is in maintenance mode (messages not rendered).

PR by @klonos: https://github.com/backdrop/backdrop/pull/1789