Improve pages when there is no organization or it has been deactivated

[alya]

This ended up being more tangly than I expected, so I'm putting up this WIP PR, which will require some work to complete.

No organization

PR screenshot

To do:

• Fill in the URL (e.g., "something.zuplichat.com")
• For self-hosted organizations, "contact Zulip support" -> "contact this Zulip server's administrators".
• Fix the CSS so that it looks like the deactivated organization page.

Deactivated organization

PR screenshot

To do:

• Split the moved organization content into a separate template, so that it can have its own title ("Organization moved"). (Edit: Added an if condition to do it in the same template)
• For self-hosted organizations, "contact Zulip support" -> "contact this Zulip server's administrators".

[zulipbot]

Hello @zulip/server-misc members, this pull request was labeled with the "area: portico" label, so you may want to check it out!

[amanagr]

[amanagr]

@alya updated!

[alya]

The screenshots look good to me!

[timabbott]

Merged, thanks @alya and @amanagr!