Some early steps in restore are using capability checks to decide how to
restore some information. But at that stage, if the course contents have
been removed and the user had perms at course level only (usually teacher)
the process can fail because some content is not restoreable due to the
lack of permissions.
Moving the restorer step to the beginning really helps is those situations
and it does not hurt at all.
Also, fixed one notice detected when testing this.
* Category + course selectors now perform required capability checks
* Show/Hide settings from UI tested and implemented for overwrite_conf setting
* Final continue button is GET rather than post
* Temp zip file is deleted after successfull extraction
* Removed previous button from settings stage MDL-23480
