Create @Theme annotation

[caalador]

There should be a @Theme annotationt that takes
as the value Class<? extends AbstractTheme> that
contains any bootstrap additions and import rewrites.

Same usage rules as for @Viewport, @Inline, @BodySize and PageConfigurator for discovering the theme.

[pleku]

Acceptance Criteria

There is a tutorial on how to apply your own theme using the annotation, and what files need to be and where. It should explain how it is different to the actual application theme, which we will add tutorial as part of another issue. The application theme should go to src/main/webapp/frontend/styles/shared-styles.html