Consolidate capture targets and allow outline path in datetree targets

Consolidate capture targets and allow outline path in datetree targets

* doc/org.texi: Document the new capture templates target.
* lisp/org-capture.el (org-capture-upgrade-templates): New function.
(org-capture-templates): Implement the new file+olp+datetree target.
(org-capture): Document the C-1 prefix.
(org-capture-sanitize-olp): New function.
(org-capture-select-template): Make sure upgraded version of templates
variable is used.
* lisp/org-datetree.el (org-datetree-find-date-create): Accept
`subtree-at-point' as a value for the KEEP-RESTRICTION argument.
(org-datetree-find-iso-week-create): Accept `subtree-at-point'
as a value for the KEEP-RESTRICTION argument.
* lisp/org.el (org-find-olp): Throw an error when called in a
buffer that is not in Org mode.
* lisp/org-capture.el (org-capture-fill-template): Use overriding time
also in `?<>' escapes.