Added mobile toggles for responsive nav

Created by: disjfa

Added a mixin for responsive navbar design.

I moved the .navbar-toggleable-lg from the .collapse to the .navbar, couse i had to add a .navbar-header for the .navbar-brand class. Don`t know if i have to use the .navbar-header how i set it up right now, But is is a start.

Example html

<nav class="navbar navbar-dark bg-inverse navbar-toggleable-lg">
    <div class="navbar-header">
        <button class="navbar-toggler hidden-xl-up" type="button" data-toggle="collapse" data-target="#navbar-lg">
            &#9776;
        </button>
        <a class="navbar-brand" href="#">Responsive navbar</a>
    </div>
    <div class="collapse" id="navbar-lg">
        <ul class="nav navbar-nav">
            <li class="nav-item active">
                <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Features</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Pricing</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">About</a>
            </li>
            <li class="nav-item">
                <a class="dropdown-toggle nav-link" href="#" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                    Dropdown
                </a>
                <div class="dropdown-menu" aria-labelledby="dropdownMenu1">
                    <a class="dropdown-item" href="#">Action</a>
                    <a class="dropdown-item" href="#">Another action</a>
                    <a class="dropdown-item" href="#">Something else here</a>
                </div>
            </li>
            <li class="nav-item">
                <a class="dropdown-toggle nav-link" href="#" type="button" id="dropdownMenu2" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                    Dropdown
                </a>
                <div class="dropdown-menu" aria-labelledby="dropdownMenu2">
                    <a class="dropdown-item" href="#">Action</a>
                    <a class="dropdown-item" href="#">Another action</a>
                    <a class="dropdown-item" href="#">Something else here</a>
                </div>
            </li>
        </ul>
    </div>
</nav>