Introduce app.use-cdn property

[php-coder]

Let's make usage of CDN configurable via dedicated property. It simplifies the testing of CSP with CDN locally.

The following places should be updated to use this property:

• mystamps/src/main/java/ru/mystamps/web/support/spring/security/SecurityConfig.java

  Line 83 in 6dcb3f4

  boolean useSingleHost = !environment.acceptsProfiles("prod");

• mystamps/src/main/java/ru/mystamps/web/support/spring/boot/ThymeleafViewResolverInitializingBean.java

  Line 76 in 6dcb3f4

  viewResolver.setStaticVariables(resourcesAsMap(productionEnv));

  (here we should pass useCdn variable as a second argument to setStaticVariables() and later pass it to ResourceUrl.exposeWebjarResourcesToView())
• all application-*.properties files should be updated as well -- CDN should be enabled only for "prod" profile
• let's use Environment.getProperty() with true as a default value (to ensure that we don't break production)