fixed sonarlint warning: created constants for return values and model attribute names

...adlocker8/budgetmaster/categories/CategoryController.java

...a/de/deadlocker8/budgetmaster/images/MediaController.java

...de/deadlocker8/budgetmaster/reports/ReportController.java

.../de/deadlocker8/budgetmaster/search/SearchController.java

...deadlocker8/budgetmaster/settings/SettingsController.java

.../java/de/deadlocker8/budgetmaster/tags/TagController.java

...8/budgetmaster/templategroup/TemplateGroupController.java

...eadlocker8/budgetmaster/templates/TemplateController.java

...ker8/budgetmaster/transactions/TransactionController.java