Prevent inserting illegal ddocs via mango

[nickva]

Mango indexes allow specifying a custom ddoc ID in the _index POST request. Previously it was possible to insert _design/ which is an illegal doc ID.

[nickva]

With the fix:

% http put $DB/db/_design%2f
HTTP/1.1 400 Bad Request

{
    "error": "illegal_docid",
    "reason": "Illegal document id `_design/`"
}

 % http post $DB/db/_index index:='{"fields":["_id"]}' ddoc:='"_design/"'
HTTP/1.1 400 Bad Request

{
    "error": "illegal_docid",
    "reason": "Illegal document id `_design/`"
}