Allow customizing URL seed

[artofwalls]

I was going to write a text about allowing the user to customize the regular expression in order to allow fetching galleries where the index is not the rightmost number but then I realized the regular expression at the top of the generated page is editable so I would suggest you to make more visually apparent that this text can be edited by the user. Currently it is a black plain text without a box around it or any indication that it can be edited.