JumbotronHeader.php

<?php
  // This file assumes that you have included the nav walker from https://github.com/twittem/wp-bootstrap-navwalker
  // somewhere in your theme.
?>
<header class="banner navbar navbar-default navbar-static-top material-static-top" role="banner">
  <div class="container">
    <div class="navbar-header material-header">
      <link href='https://fonts.googleapis.com/css?family=Roboto:400,900,700,300' rel='stylesheet' type='text/css'>
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target=".navbar-collapse">
        <span class="sr-only"><?= __('Toggle navigation', 'sage'); ?></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>

    </div>

    <script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?sensor=false"></script>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css">

    <!-- Navigation -->
    <nav class="collapse navbar-collapse" role="navigation">
      <?php
      if (has_nav_menu('primary_navigation')) :
        wp_nav_menu(['theme_location' => 'primary_navigation', 'walker' => new wp_bootstrap_navwalker(), 'menu_class' => 'nav navbar-nav material-nav']);
      endif;
      ?>
    </nav>

    <?php 
    $src = wp_get_attachment_image_src( get_post_thumbnail_id(get_the_id()), 'large' );
    $url = $src[0];

    ?>
  </div>
  
  <!-- Jumbotron -->
  <div class="jumbotron" style="
    background: url('<?php echo $url; ?>'); 
    background-size: 
    cover;
    height: 350px;
    opacity: 1;
    background-color: black;
    color: white;">
    <div class="container">

      <!-- Change per page -->
      <?php if (is_archive()): ?>

        <h1 class="main-page-title"><?php echo single_tag_title(); ?> </h1>
      <?php else: ?>
        <h1 class="main-page-title"><?php echo the_title(); ?> </h1>
        <p><?php echo get_post_custom_values('page_subtitle', get_the_id())[0]; ?> </p>  
      <?php endif ?>
      
    </div>
  </div>

</header>

From what I read I think you should use wp_enqueue_style and wp_enqueue_script for adding styles/scripts.

I feel like I'm sticking it to the man when I use other people's servers to load resources.

You can still load it from other people's servers with the api. 💯