[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