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..
Make config items fully editable

fixes #694
Remove check on undefined module and key

To prevent module and key to be saved with values “undefined”, the
values are normalized to empty strings in the form before getting
passed.
Don't create new entry on change.

Bring back update function to delete entry when module or key name have
not changed.
Merge branch 'ac/admin_edit_config'

Fixes #700.
core: Set z_pid cookie only when set_persistent is used. Fixes #701
zotonic_status: Add 'admin' button to sites overview

See here: http://stick.im/s/pNa
mod_admin: Add dropdown to set the number of items per page on the overview page

Fixes #703
docs: move tech design doc into its own directory.
mod_admin_identity: fix identity insert race condition by moving email identity check into the #rsc_update event instead of the async #rsc_update_done event.
core: let gettext show an error if a po file could not be read (instead of crashing)
core: emqtt, use the standard Erlang stdlib 'eacces' as error.