Stop caching index HTML when server mode is development
Stop caching index HTML when server mode is development

Previously, we were always caching indexHtml. This meant that if a
new file were added (pluginCss or pluginJs) it wouldn't be reflected
(even after rebuilding with npm) unless the webserver was
restarted. This commit stops caching the indexHtml if the Girder
installation is in development mode.

RafaelPalomar/girder,Kitware/girder,data-exp-lab/girder,adsorensen/girder,RafaelPalomar/girder,sutartmelson/girder,jbeezley/girder,sutartmelson/girder,data-exp-lab/girder,Xarthisius/girder,kotfic/girder,girder/girder,girder/girder,kotfic/girder,girder/girder,Kitware/girder,manthey/girder,adsorensen/girder,manthey/girder,RafaelPalomar/girder,RafaelPalomar/girder,data-exp-lab/girder,Kitware/girder,sutartmelson/girder,data-exp-lab/girder,jbeezley/girder,kotfic/girder,Xarthisius/girder,adsorensen/girder,manthey/girder,jbeezley/girder,kotfic/girder,manthey/girder,sutartmelson/girder,kotfic/girder,data-exp-lab/girder,sutartmelson/girder,Xarthisius/girder,girder/girder,Xarthisius/girder,Kitware/girder,Xarthisius/girder,adsorensen/girder,RafaelPalomar/girder,adsorensen/girder,jbeezley/girder