Ability to set header image for a Page (#6210)

* Ability to set header image for a Page

 - Add header image to Page
 - Show it on Page view
 - Show correctly it on Page list view
 - On the Page list view, pages have a light border
   to make it easier to see an image belongs to a page

* Maybe it looks better

* Use <img> instead if <x-image>

* src -> :src; set width

* Update page.vue

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>