move check_canedit, check_content to IkiWiki library from editpage

move check_canedit, check_content to IkiWiki library from editpage

It no longer makes sense to keep these functions in editpage, because
serveral plugins now exist that use them, and users may want to disable
editpage, while leaving those plugins enabled.

Most notably, comments uses both functions, and it's entirely appropriate
to disable editpage but still want to have comments enabled.

Less likely, attachments, rename, and remove all use check_canedit -- but
it would be unusual indeed to want to use these w/o editpage.