Add project description field.

Description is shown in a popup when hovering the 'information' icon, like column description,
which is shown in project list on dashboard and in board's title.

This is useful to document the categories and colors used in the project.

app/Template/project/edit.php

@@ -9,7 +9,28 @@
     <?= $this->formLabel(t('Name'), 'name') ?>
     <?= $this->formText('name', $values, $errors, array('required', 'maxlength="50"')) ?>
 
+    <?= $this->formLabel(t('Description'), 'description') ?>
+
+    <div class="form-tabs">
+
+        <div class="write-area">
+          <?= $this->formTextarea('description', $values, $errors) ?>
+        </div>
+        <div class="preview-area">
+            <div class="markdown"></div>
+        </div>
+        <ul class="form-tabs-nav">
+            <li class="form-tab form-tab-selected">
+                <i class="fa fa-pencil-square-o fa-fw"></i><a id="markdown-write" href="#"><?= t('Write') ?></a>
+            </li>
+            <li class="form-tab">
+                <a id="markdown-preview" href="#"><i class="fa fa-eye fa-fw"></i><?= t('Preview') ?></a>
+            </li>
+        </ul>
+    </div>
+    <div class="form-help"><a href="http://kanboard.net/documentation/syntax-guide" target="_blank" rel="noreferrer"><?= t('Write your text in Markdown') ?></a></div>
+
     <div class="form-actions">
         <input type="submit" value="<?= t('Save') ?>" class="btn btn-blue"/>
     </div>
-</form>
+</form>