Use Sass to style search form, remove search template

[retlehs]

this PR removes the custom search template in favor of the html5 searchform from wp core. this also allows us to drop lib/utils.php. some basic form styling is added with sass, but it's different than the current searchform.

existing markup:

<form role="search" method="get" class="search-form form-inline" action="http://example.dev/">
  <label class="sr-only">Search for:</label>
  <div class="input-group">
    <input type="search" value="" name="s" class="search-field form-control" placeholder="Search Example Site" required>
    <span class="input-group-btn">
      <button type="submit" class="search-submit btn btn-default">Search</button>
    </span>
  </div>
</form>

new markup (from wp core):

<form role="search" method="get" class="search-form" action="http://example.dev/">
  <label>
    <span class="screen-reader-text">Search for:</span>
    <input type="search" class="search-field" placeholder="Search …" value="" name="s" title="Search for:">
  </label>
  <input type="submit" class="search-submit" value="Search">
</form>

existing searchform:

new searchform: