Item12783: trimmed-down first time configure

Get rid of many UI elements not needed during first-time configure:

   * error/warnings/unsaved summary up top
   * detailed error list in extra tab
   * navigation tabs (we'd left with two tabs at this point)
   * a few easily guessable paths, by making them expert settings and adding support for "silently" guessed path settings
   *

Also shorten the welcome message down to the bare essentials, making it a
little bit friendlier in the process, and remove redundant hints from the
intro in Foswiki.spec.


git-svn-id: http://svn.foswiki.org/trunk@17356 0b4bb1d4-4e5a-0410-9cc4-b2b747904278