[keycloak-dev] Loading message bundles from themes

You should load message bundles from themes not directly from the file system. That's how login pages and account management loads messages bundles so it should be consistent, but more importantly doing it through themes gives the following: * Support for loading from file or classpath - we also have an SPI for theme loaders so they can in the future load resources from other sources as well, for example the database * Supports overriding messages from themes - users can define custom themes that are used on a per-realm basis that can override messages. Themes even inherit messages from the theme it extends, so can choose to override only some messages Themes expects the message bundle to be named "messages_<locale>.properties". I'd prefer it to be consistent between the 3 different things we have internationalized and such admin messages should be loaded from themes and the message bundles should have the same names. We can then discuss with the translation team if they are happy with 3 separate message bundles or if they'd like a single message bundle for everything. We can also query about whether or not we can divide the message bundles up further. Dividing message bundles up would require adding support for that to themes as well. With classloaders and such it would be hard to implement a list available bundles so that's another reason for going with a single message bundle for now.