admin: don't enable do_cropcenter if there is no media img element.
core: use z_trans:lookup_fallback/2 from compiled templates.

https://github.com/zotonic/zotonic/commit/51b09f3339322efcb42cf2d290f18deb18055e7e#commitcomment-4757206
core: fix bug in z_trans:translations/2 when called with a {trans, ...} tuple.

(cherry picked from commit c721221d435c9cd39346f4382864060a110a56a6)
mod_base: controller_page - check if page path equals the request path

This change lets controller_page check if we need to be at a different
URL when the page path of a resource is set.

If set, we need to redirect there if the current request's path is not
equal to the resource's path. This prevents duplicated content on
multiple URLs.
mod_base: controller_page - make canonical redirect behavior configurable.
core: Removed ancient leftover.
admin_frontend_edit: remove x-character after close button title.
core: fix possible timeout error in mediaclass file scanning.
doc: Clarify the doc page about custom pivots
mod_export: add #export_resource_visible{} notification. Lists of data are now encoded as csv by default..
Merge branch 'master' of https://github.com/ArthurClemens/zotonic

Conflicts:
	include/zotonic_notifications.hrl
	modules/mod_admin/lib/js/modules/z.cropcenter.js
	modules/mod_admin_frontend/templates/page_admin_frontend_edit.tpl
	modules/mod_export/controllers/controller_export_resource.erl
	modules/mod_export/mod_export.erl
	src/support/z_mediaclass.erl