[GEOS-6683] Ignore numberMatched per layer

This commit adds a value to the user- and rest interface for disabling
counting the numberMatched property. On large queries with very few
matches this can cause a significant speed boost.
[GEOS-6683] Ignore numberMatched per layer test

Extra commit with an additional, required test for the above issue.
[GEOS-6683] Ignore numberMatched per layer, description

Added a help modal with an additional help text to assist GeoServer
admins on how and when to use this feature.
Rework of patch GEOS-6655: Add text/xml MIME-types to WMS
Extra set the offset to 0 to disable paginating when skipping the NumberMatched element. This creates a faster request.
[GEOS-6683] Migrated the type of numberMatch back to BigInteger from String. Appearently GeoTools already provides parsing of an unknown value when null is provided. See also: https://github.com/geotools/geotools/commit/bb67ae0d41c1143d8f70bbc56c728c9b0c488e84
[GEOS-6683] Forgot a typo and class.
[GEOS-6683] Type migrated GeoJSON

I migrated the type from GeoJSON back to an Integer value instead of a
String, this is in line with the new approach that Andrea and Justin
advised me to do. Also I removed BigInt, no idea why IntelliJ put it
there.

Thank you all for your patience with me!
[GEOS-6683] Forgot an exclamation mark

Sorry guys, I am a real idiot. The build should pass all tests now.
Merge pull request #768 from Geodan/GEOS-6683

[GEOS-6683] Ignore numberMatched per layer
Merge pull request #775 from Geodan/GEOS-6655-v2

Rework of patch GEOS-6655: Add text/xml MIME-types to WMS
Merge branch 'master' of git://github.com/geoserver/geoserver