"Dropdown" class needs specificity

[tigerhawkvok]

As per: https://www.polymer-project.org/docs/elements/paper-elements.html#paper-dropdown-menu

<paper-dropdown-menu label="Your favorite pastry">
    <paper-dropdown class="dropdown">
        <core-menu class="menu">
            <paper-item>Croissant</paper-item>
            <paper-item>Donut</paper-item>
            <paper-item>Financier</paper-item>
            <paper-item>Madeleine</paper-item>
        </core-menu>
    </paper-dropdown>
</paper-dropdown-menu>

Currently, if Bootstrap is installed, at-odds definitions in Bootstrap and the requirement of the "Dropdown" class create conflicts. In particular, Bootstrap overrides the position: absolute.

Even doubling up the definition of html /deep/ paper-dropdown to html /deep/ paper-dropdown, html /deep/ paper-dropdown.dropdown should fix this.