Import config generation script from Gist

Import gettingStartedConfigGenerator.php from
https://gist.github.com/phuedx/8941767 with the following modifications:

* Convert it to a maintenance script
* Generate values for the wgGettingStartedCategoriesForTaskTypes and
  wgGettingStartedExcludedCategories configuration variables
* The list of DB names (see $wmgUseGettingStarted in
  gettingStartedConfigGenerator.php) is passed via the dbnames option.
  If the list of DB names is passed, then entries in the configuration
  variables are filtered by that list

Change-Id: I708753d298c8eb04a687d07afcd5833ad8da68ba
(cherry picked from commit 6d5c9fb3f4e300b0723819da2cd7365e82dabef4)