[REF] ir_qweb: less webasset: compress output instead of minifying it

* Remove dependency on less-plugin-clean-css which distribution
  packages are broken on debian stretch and derivated.
* Updated documentation.
* Lessc provides a `--compress` argument that could solve this issue.
  According to the documentation at http://lesscss.org/usage/: "[...]
  Compress using less built-in compression. This does an okay job but
  does not utilise all the tricks of dedicated css compression[...]"
  However, using this argument works on lessc 2.5.3 but not on 1.4.2
  (`--compress` arg is present but seems to be break some rules
  i haven't been able to put my finger on).
* Finally, use the minification of the StylesheetAsset that takes care
  of removing the spaces, the comments and the sourcemap with regexes.

fixes issue #9113

debian/control

@@ -15,7 +15,6 @@ Depends:
  ${misc:Depends},
  adduser,
  node-less,
- node-clean-css,
  postgresql-client,
  python,
  python-dateutil,

doc/setup/install.rst

@@ -462,25 +462,25 @@ Source installation requires manually installing dependencies:
             $ apt-get install -y npm
             $ sudo ln -s /usr/bin/nodejs /usr/bin/node
 
-    Once npm is installed, use it to install less and less-plugin-clean-css:
+    Once npm is installed, use it to install less:
 
     .. code-block:: console
 
-        $ sudo npm install -g less less-plugin-clean-css
+        $ sudo npm install -g less
 
   - on OS X, install nodejs via your preferred package manager (homebrew_,
-    macports_) then install less and less-plugin-clean-css:
+    macports_) then install less:
 
     .. code-block:: console
 
-        $ sudo npm install -g less less-plugin-clean-css
+        $ sudo npm install -g less
 
   - on Windows, `install nodejs <http://nodejs.org/download/>`_, reboot (to
-    update the :envvar:`PATH`) and install less and less-plugin-clean-css:
+    update the :envvar:`PATH`) and install less:
 
     .. code-block:: ps1
 
-        C:\> npm install -g less less-plugin-clean-css
+        C:\> npm install -g less
 
 Running Odoo
 ------------

openerp/addons/base/ir/ir_qweb.py

@@ -1636,9 +1636,6 @@ class PreprocessedCSS(StylesheetAsset):
         self.html_url_format = '%%s/%s/%%s.css' % self.bundle.xmlid
         self.html_url_args = tuple(self.url.rsplit('/', 1))
 
-    def minify(self):
-        return self.with_header()
-
     def get_source(self):
         content = self.inline or self._fetch_content()
         return "/*! %s */\n%s" % (self.id, content)
@@ -1688,7 +1685,7 @@ class LessStylesheetAsset(PreprocessedCSS):
         except IOError:
             lessc = 'lessc'
         lesspath = get_resource_path('web', 'static', 'lib', 'bootstrap', 'less')
-        return [lessc, '-', '--clean-css', '--no-js', '--no-color', '--include-path=%s' % lesspath]
+        return [lessc, '-', '--no-js', '--no-color', '--include-path=%s' % lesspath]
 
 def rjsmin(script):
     """ Minify js with a clever regex.

setup.cfg

@@ -6,7 +6,6 @@ requires =
   babel
   libxslt-python
   nodejs-less
-  nodejs-clean-css
   pychart
   pyparsing
   python-dateutil

setup/package.dfcentos

@@ -10,7 +10,6 @@ RUN yum install -d 0 -e 0 epel-release -y && \
 		babel \
 		libxslt-python \
 		nodejs-less \
-		nodejs-clean-css \
 		pychart \
 		pyparsing \
 		python-dateutil \

setup/package.dfdebian

@@ -18,7 +18,6 @@ RUN apt-get update -qq &&  \
 	apt-get install \
 		adduser \
 		node-less \
-		node-clean-css \
 		postgresql \
 		postgresql-client \
 		python \

setup/package.dfsrc

@@ -21,7 +21,6 @@ RUN apt-get update -qq &&  \
 		postgresql-client \
 		adduser \
 		node-less \
-		node-clean-css \
 		libxml2-dev \
 		libxslt1-dev \
 		libldap2-dev \