mod_admin: Only show "view" button when rsc has a page_url.

Otherwise, clicking the "view" button does not do anything
mod_admin: fix for tab content getting scrollbars inside modal
doc: Document API services
doc: Add more links to the glossary.
doc: Remove templates overview from index
mod_comment: Add link to settings page
doc: Add glossary link to API services manual
doc: Add some single-line (or more) "blurb" to each controller
doc: Document some more actions ("blurb" only)
doc: add chapter about authentication.
doc: add some seealso refs to the controller_log{on|off}.
doc: add basic authorization docs.
doc: fix see also refs in datamodel chapter.
mod_oembed: Add maxwidth/maxheight options; add documentation.
core: restructure and improve acl routines (issue #463).

It started off with adding a guard to z_acl:wm_is_authorized/2.
Along with that I wanted to add some -spec's for that family of functions,
which led to adding -type's in webzmachine and m_rsc.

Also, it felt backwards to require the id to be integer at this stage,
which turned out to affect the acl modules handling the authorization.

Also, m_rsc:get_acl_props/2 always returns a #acl_props{} record when
given a integer id, even if no resource with that id exists, so to be
consistent I updated it so that also applies when presented with a
resource name.

This is a rather big chunk of changes in one commit, but they are rather
entangled, at least when applied in the order I made them.
mod_acl_adminonly: missed to remove one is_integer(Id)-guard.
core: Add category permission check to m_media and check for invalid URLs
mod_admin: Improved media upload feedback; dont close dialog on failure.
mod_admin: Show currently logged in user on dashboard.
mod_survey: add option to show true/false and yes/no as buttons.