Introduce command to install multisite from scratch

Prompted by #486.

Old:

* `core install-network`

New:

* `core multisite-install`
* `core multisite-convert`

Tasks:

* [x] add Behat scenarios
* [x] get rid of warnings caused by early `get_site_option()` calls:

    * ms_files_rewriting
    * active_sitewide_plugins
    * siteurl
    * WPLANG
    * _site_transient_update_core
    * _site_transient_update_themes
    * _site_transient_update_plugins
* [x] use SUBDOMAIN if it's defined; use $base, if it's defined, etc.
* [x] add docs
* [x] test interaction with `wp core is-installed`