title: Substitute a Rails form submit button by a Bootstrap icon
body: <p>I have an app in which users and groups are linked by a many-to-many relationship.</p>
  
  <p>I have a join table GroupUsers, and I would like to enable an user to add or remove a group from his directly in the group index, which is displayed as a table.</p>
  
  <p>To add a group, I have added a form in one of the column of the index that creates a GroupUser entity. The problem is, I would like to use a Bootstrap icon-plus-sign to submit the form (the user_id and group_id are in hidden fields). </p>
  
  <p>My column looks like this (in slim, <code>.</code> is substitute for <code>class=</code> in HTML parts) :</p>
  
  <pre><code>td
    = form_for @item, as: :group_user, url: "groups/#{group.id}/users" do |f|
      = f.hidden_field :user_id, :value =&gt; current_user.id
      = f.hidden_field :group_id, :value =&gt; group.id
      .form-actions
        = f.submit '' do 
          i.icon-plus-sign.icon.icon-large
  </code></pre>
  
  <p>For the moment, instead of only a plus sign I get an empty button in the table, what shall I do ?</p>
  
  <p>PS: I've checked this answer: <a href="https://stackoverflow.com/questions/9376192/add-icon-to-submit-button-in-twitter-bootstrap-2">Add icon to submit button in twitter bootstrap 2</a>, which includes a bootstrap icon inside the button, but I want to have the icon instead of the button.</p>