fixing webstart index page that allows to set custom intro page for OMERO.web
removing unnecessary import
adding default template
adding never_cache decorator to always create a 302
and not cache webstart page
making TEMPLATE_DIRS configurable in settings
updating webstart url pattern
making INDEX_TEMPLATE configurable to define intro page
moving custom index page to the top
changing year in copyrights
Merge pull request #2239 from aleksandra-tarkowska/rebased/dev_5_0/webstart_index_template

 Webstart index template (rebased onto dev_5_0) #2210