CORE-6316 Converted AppTemplateUtils to singleton

Methods no longer used statically. Classes which previously used
AppTemplateUtils were updated to use the new singleton AppTemplateUtils
class via dependency injection (both manual and via Gin).
CORE-6316 Converted DiskResourceUtil to singleton
CORE-6316 Converted CommonModelUtils to singleton
CORE-6316 Converted JsonUtil to singleton
CORE-6316 Converted SearchModelUtils to singleton
CORE-6316 Converted CollaboratsUtil to singleton
CORE-6316 Code cleanup