Show a tip at the end of the installer to prompt about extensions

Almost every wiki install is going to want one or more extensions to
enhance their users' experience. Give the wiki admin as much info
as possible so they can find extensions that may be useful to them.

This is shown as an info box on the final page of installation.

Bug: 43504
Change-Id: Iba5d53a485d7b0b04cb9104d7d964fc01a1c1cae
Merge "Show a tip at the end of the installer to prompt about extensions"