[zmq] add an introductive docstring on ZMQComm

[server config] on quick start, system source is still activated. Closes #2535714

[web] use `inline` `Content-Disposition` by default (closes #2535734) since known time we have been serving static file with a `filename` parameter on the `Content-Disposition` header. But since d74addac92bb we explicitly serve file as attachment if a filename is provided. However this is valid to have `inline` disposition and `filename` parameter. This changeset revert this part of d74addac92bb, going back to `inlined` by default. `IDownloadable` code explicitly request `attachment` content to preserve expected behavior.

[web] allow configuration of the Content-disposition value The `set_content_type` function now takes and optional `disposition` parameter to control the value of this HTTP header. Use of `Content-disposition: inline` with a filename parameter are valid, so the presence of filename does not allows to choose between `attachment` and `inline`

[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