New 'newimagestext' message on [[Special:NewFiles]]

New 'newimagestext' message on [[Special:NewFiles]]

There is a default message called 'newimages-summary' but its output
is in user language and not in content language. Therefore, to customize
the introduction at [[Special:NewFiles]], one has to override all trans-
lations (which in turn must be maintained).

Use cases:
* Wikimedia Commons: Wants to add links to external tools and different
  gallery styles.
  Currently [[:commons:MediaWiki:Newimages-summary]] is used, and thus
  the customization is only visible to users with English user language
  preference set.

Change-Id: I4a253847d25540c90a0708636058ca821b035b06