For some API use cases, it will certainly be helpful to find out which modules are enabled in a project. It is at least for a new feature we're developing at "Planio":https://plan.io/redmine-hosting. But in general, every API client which uses - let's say the issues API - should be able to check beforehand, if the issue tracking module is active in a particular project or not.

This patch adds @enabled_modules@ to the @projects#show@ API. They will be shown only if the @include=enabled_modules@ URL parameter is used. The behaviour is exactly the same as for issue categories and trackers.

And while we're at it, it also makes the @include=...@ URL parameters (for issue categories, trackers and enabled modules) available in the @projects#index@ API.

Tests are included, only needs merging :)