#6944072 Adds a page for in-progress DIFM sites

Changes proposed in this Pull Request

Adds a new page to show the in-progress status of a DIFM Lite site. The page is shown if the site is currently being built by the DIFM Lite Team. Please see D68203-code for more details.

Also blocks access to site editing features in the admin menu. The page and blocking is similar to the ones shown for domain-only sites.

If there is an active support session, the page is not shown and menus are not hidden.

Please see pdh1Xd-4l-p2 for additional context.

Testing instructions

• Apply D68203-code
• Enable the difm-lite-in-progress sticker using Blog RC.
• Switch to the site with the sticker enabled and check the UI:
  

• Start a support session.
• In the incognito window started by the support session, open calypso.localhost or the calypso.live link in a new tab.
• Confirm that the above page is not shown.

Pending

EDIT: These tasks will be done in a future iteration.

• Change the name.
• Update the design of the page with feedback received from .