Remove dependency on Less extension; work around lessphp bug

Less files are supported by MW core from 1.22 onwards.

lessphp has a bug where certain ways of calling mixins are not supported (see https://github.com/leafo/lessphp/issues/481).
This change points the bootstrap submodule to a fork of the bootstrap repo (https://github.com/s7eph4n/bootstrap.git) where this bug is not present.
As soon as the underlying bug in lessphp is fixed and available in mw core the submodule will be pointed at the official bootstrap repo (https://github.com/twbs/bootstrap.git) again.

Change-Id: Ie5906ebc28560333115837d50cd6a486866e283a