memberships: modal for confirmation (#130)

* unpolished version of working modal

* separated JS script

* minor change in JS

* minor changes JS file
diff --git a/core/auth/memberships/group.html b/core/auth/memberships/group.html
index 3995fb7..42a2e38 100644
--- a/core/auth/memberships/group.html
+++ b/core/auth/memberships/group.html
@@ -27,7 +27,7 @@
     <hr class="divider">
     <form action="/group/{{ .GroupName }}/add-child-group" method="post">
         <label for="child-group">Select Child Group:</label>
-        <select id="child-group" name="child-group" required>
+        <select id="child-group" aria-label="Select" name="child-group" required>
             {{- range .AvailableGroups }}
             <option value="{{ . }}">{{ . }}</option>
             {{- end }}
@@ -44,8 +44,8 @@
         <tr>
             <td><a href="/user/{{ . }}">{{ . }}</a></td>
             <td>
-                <form action="/group/{{ $parentGroupName }}/remove-owner/{{ . }}" method="post" onsubmit="return confirm('Are you sure you want to revoke user {{ . }} ownership of the {{ $parentGroupName }} group?')">
-                    <button type="submit" class="button">Remove</button>
+                <form action="/group/{{ $parentGroupName }}/remove-owner/{{ . }}" method="post" class="remove-form" data-confirmation-message="Are you sure you want to revoke user <strong>{{ . }}</strong>'s ownership of the  <strong>{{ $parentGroupName }}</strong> group?">
+                    <button type="submit">Remove</button>
                 </form>
             </td>
         </tr>
@@ -61,7 +61,7 @@
         <tr>
             <td><a href="/user/{{ . }}">{{ . }}</a></td>
             <td>
-                <form action="/group/{{ $parentGroupName }}/remove-member/{{ . }}" method="post" onsubmit="return confirm('Are you sure you want to remove user {{ . }} user from {{ $parentGroupName }} group?')">
+                <form action="/group/{{ $parentGroupName }}/remove-member/{{ . }}" method="post" class="remove-form" data-confirmation-message="Are you sure you want to remove user  <strong>{{ . }}</strong> user from  <strong>{{ $parentGroupName }}</strong> group?">
                     <button type="submit" class="button">Remove</button>
                 </form>
             </td>
@@ -93,12 +93,23 @@
             <td><a href="/group/{{ .Name }}">{{ .Name }}</a></td>
             <td>{{ .Description }}</td>
             <td>
-                <form action="/group/{{ $parentGroupName }}/remove-child-group/{{ .Name }}" method="post" onsubmit="return confirm('Are you sure you want to remove group {{ .Name }} as a child of the group {{ $parentGroupName }}?')">
+                <form action="/group/{{ $parentGroupName }}/remove-child-group/{{ .Name }}" method="post" class="remove-form" data-confirmation-message="Are you sure you want to remove group  <strong>{{ .Name }}</strong> as a child of the group  <strong>{{ $parentGroupName }}</strong>?">
                     <button type="submit" class="button">Remove</button>
                 </form>
             </td>
         </tr>
         {{- end }}
     </table>
+    <dialog id="confirmation" close>
+        <article>
+            <h2>Confirm Your Action</h2>
+            <p id="confirmation-message">Are you sure?</p>
+            <footer>
+                <button id="cancel-button" class="secondary cancel-button">Cancel</button>
+                <button id="confirm-button">Confirm</button>
+            </footer>
+        </article>
+    </dialog>
+    <script src="/static/main.js"></script>
 </body>
 </html>