Add a "Contact support" link to help menu

[alya]

We should add a "Contact support" link to the help menu (in the upper right of the app):

• Make /support work on self-hosted servers.
• Add a note at the top of the content area explaining who you are contacting:
  • Zulip Cloud: "Your message will be sent to Zulip Support."
  • Self-hosted: "Your message will be sent to this Zulip server's administrators at [<admin email for the server>](mailto:<admin email for the server>)."
    
• Add a "Contact support" link to /support at the bottom of the menu, after a divider line.

While we're looking at this, we should also make some tweaks to the /support form:

• Change From value to "Name "
• Change Organization -> Zulip URL, and show the organization's URL.

The generated email should be updated accordingly.

[zulipbot]

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