Create a more specific method to handle translations joins

[emiliemarchand]

In the TranslatableEntityRepository, the getCurrentAppName() method is used to check whether or not the translations table should be inner joined or left joined. However, since this method is a bit generic it could be used in the future for other things, but is not currently available in repositories of non-translatable entities.

I suggest we move it into the BaseEntityRepository, which is common to both. And we could use a more specific method available only in the TranslatableEntityRepository to handle the joins. Something like set/getJoinType(). That way, left joins would not be exclusive to the backend app. (Although I don't see another use case at the moment)

For now, I created a quick workaround to fix a bug in the locale switcher by checking if the repository has the translatable trait, but it won't be required anymore if we proceed to those changes. (See this pull request: #144)