Fix for compatibility with bootstrap 4.0.0 alpha 6

[jcoyne]

Build is currently failing with:

       Undefined variable: "$font-size-root".
     # ./app/assets/stylesheets/blacklight/_icons.scss:3
     # ./app/assets/stylesheets/blacklight/_blacklight_base.scss:24
     # ./app/assets/stylesheets/blacklight/blacklight.scss:5
     # ./.internal_test_app/app/assets/stylesheets/blacklight.scss:3

Additionally, alpha 6 drops the -xs which we are using:

app/views/catalog/_facet_index_navigation.html.erb:  <%= link_to t('blacklight.search.facets.clear'), @pagination.params_for_resort_url('index', search_state.to_h.except(@pagination.request_keys[:prefix])), data: { blacklight_modal: "preserve" }, class: ['btn btn-secondary btn-xs', ('disabled' unless @pagination.prefix.present?)].join(' ') %>
app/views/catalog/_facet_index_navigation.html.erb:  <ol class="pagination pagination-xs">
app/views/catalog/_facets.html.erb:  <div id="facet-panel-collapse" class="collapse navbar-toggleable-xs">
app/views/shared/_header_navbar.html.erb:    <button class="navbar-toggler hidden-xs-up" type="button" data-toggle="collapse" data-target="#user-util-collapse" aria-controls="user-util-collapse" aria-expanded="false" aria-label="Toggle navigation"></button>
app/views/shared/_header_navbar.html.erb:    <div class="collapse navbar-toggleable-xs float-md-right" id="user-util-collapse">

We can also drop the .nav base class from:

app/views/catalog/_home_text.html.erb:  <nav class="nav nav-pills">
app/views/catalog/_show_tools.html.erb:    <ul class="nav nav-pills nav-stacked">
app/views/shared/_user_util_links.html.erb:<ul class="nav navbar-nav">