Improve prerequisite handling of partials/templates

Now each file in app-prod depends only on the corresponding
partial of the app directory using a static pattern rule.
No need for APP_TEMPLATES_SRC in Makefile

And APP_PROD_TEMPLATE_FILES is a better name than APP_TEMPLATES_DEST. More consistent with the other variable names.
Merge pull request #88 from geoadmin/dev_make_partials_improve

Improve prerequisite handling of partials/templates