Use autoless to compile LESS files

autoless is a .less file watcher I put together some time ago and
improved recently. This changeset will not change anything for people
who don't deal with .less files.

For those who do:
* make less is _much_ faster (and it still runs before every commit when
  .less files change)
* there's no need for modifying Makefile when a new .less file is added
  to the project
* you can run make lesswatch and it will keep monitoring .less files for
  changes and generate appropriate .css files if needed (it tracks
  dependencies, so if you change e.g. mf-mixins.less, everything that
  depends on it will get recompiled)

It also adds make installhooks to install all the necessary git hooks
(the new hooks pause lesswatch while doing git rebase so that git
doesn't get confused by .css files being changed during a rebase).

package.json file is used to make sure that appropriate Node.js modules
are installed in MobileFrontend's directory, not parent directories.

The small change in watchlist.css is a result of less not being run
correctly in https://gerrit.wikimedia.org/r/#/c/48861/

Change-Id: I0f095ec206901ea9bad45903953c66d2611311d9
Merge "Use autoless to compile LESS files"