Simplify app_reverse quite a bit

Refs #462, #379, #306.

The code using proximity_info to find the best application content is really convoluted and also has a few bugs such as not taking into account the `active` flag of pages. I propose a massive simplification of the `app_reverse` mechanism where by default only the current language is taken into account for choosing the best matching  application content.

We only ever used the same application content several times on one site if we had several languages at the same time, and never used the same application content in the same language on a site.

Opinions?