Press landing page should use subdomain

[jcoyne]

Extracted from #93 and Blocked by #56

Acceptance criteria

• There's a page at <subdomain>.<main-press-fqdn>
• We may also need to host a press completely under its own domain (e.g. leverpress.org).

[acozine]

Updated #56. Is that all we need to unblock this story?

[jcoyne]

Yes, this is now unblocked.

[sethaj]

AC: When a user is on a press subdomain, search works only within that subdomain.

[jgmorse]

To have a press under its own domain (not a subdomain of fulcrum.org), would the same solution apply?

[mbakeryo]

We need to raise this for HEB, Lever, UMPRE though.