[downloadable] fix filename HTTP header for simple name with space (closes #2535715) Since d74addac92bb, we export simple ascii filename without any encoding in the `filename` parameter of the `Content-Disposition` header. If this name contains space this will fails, the parameter value will be truncated at the space position. (eg. `filename=jungle babar.txt` read as `jungle`) We need to quote the filename to prevent this (eg. `filename="jungle babar.txt"`). Then literal quote and backslash needs to be escaped too. The new escaping is correct according this extensive test case data base: http://greenbytes.de/tech/tc2231/

merge fixes from stable

Added tag cubicweb-debian-version-3.15.6-1 for changeset b05e156b8fe7

Added tag cubicweb-version-3.15.6 for changeset 0163bd9f4880

prepare 3.15.6

[web] add a digital signature to error form (closes #2522526) Simple (and quite weak) implementation of a digital signature of the content to be submited by email in the error report view generated by ErrorView. The signature is a simple hmac hash computed using a secret key (generated at repository startup) and the "secret" form content to be included in the notification email. The controller can then check this content has not been modified or forged by a malicious user.

[web/views] bugfix: the mime type is text/plain, not text/txt (closes #2526345)

[doc] fix of personnal etc directory in book

[web] add a Forbidden exception This is similar to the Unauthorized exception, but generates a 403 error instead of a 401 (Unauthorized)

[web] add a ``anonymize-jsonp-queries`` option in file configuration (closes #2465388) This option controls connection anonymizing before executing any query for CSRF / safety reason.