- Added useHeadersProxyURL parameter to the global settings page
- Refactored ProxifyingURLMangler.java to follow use the Proxy Base URL
  only when the useHeadersProxyUERL is not set

…GEOS-8240

- Added template expansion with QuickTemplate
- Added headers of interest to the template
- Added more than one template in the smae base proxy URL

- Adde case-insensitive header names template expansion

- Added a check against base proxy URL being null
- Dropped base proxy URL validator in the gloabl settings page
  (template expressions do not pass the URL validator, of course)

- Used enum in lieu of static members to hold header names

- Used enum in lieu of static members to hold header names

- Added "Forwarded" header the templates
- For simmmetry with the "Forwrded-Path" header, a non-standard
  "path=<path value" header value component  is supprtorted.
  For example, this template
  ${Forwarded.proto}://${Forwarded.host}/${Forwarded.path}/geoserver
  can be expanded with this header
  Forwarded: proto=http; host=example.com:8080; path=public
  to
  http://example.com:8080/public/geoserver

- Corrected copyright years in headers
- Reverted ColorMapLegendCreatore.java to its original state

- Reverted ColorMapLegendCreatore.java to its original state