Validate color existence when setting project color

Summary:
Do not allow setting an invalid project color via the `project.edit` Conduit API but validate the value.

Same game as in D26430, D26459.

Closes T16237

Test Plan:
* Run `echo '{"transactions":[{"type":"name","value":"projectWithInvalidColor"},{"type":"color","value":"purpleyellowgreyish"},{"type":"description","value":"project project new new new"}]}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" project.edit --`
* Succeed before applying this patch
* Fail after applying this patch:
```
{
"error": "ERR-CONDUIT-CORE",
"errorMessage": "ERR-CONDUIT-CORE: <project.edit> Validation errors:\n - Value for \"project:color\" is invalid: \"purpleyellowgreyish\".",
"response": null
}
```
* Still succeed with a valid name like `{"type":"color","value":"checkered"}`
* Manually set a color in the project using the frontend. It works.
* Change every single other color, like Red, Orange, Yellow, ... Grey, Checkered, they all works
* Set color to Orange. Archive the project. Activate the project. It still works, you still see the original color Orange.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16323, T16237

Differential Revision: https://we.phorge.it/D26460

src/applications/project/xaction/PhabricatorProjectColorTransaction.php

···
       $this->renderValue(PHUITagView::getShadeName($new)));
   }
 
+  public function validateTransactions($object, array $xactions) {
+    $errors = array();
+
+    if (!$xactions) {
+      return $errors;
+    }
+
+    foreach ($xactions as $xaction) {
+      $new_color = $xaction->getNewValue();
+      if (!PhabricatorProjectIconSet::getColorName($new_color)) {
+        $errors[] = new PhabricatorApplicationTransactionValidationError(
+          self::TRANSACTIONTYPE,
+          pht('Invalid'),
+          pht(
+            'Value for "%s" is invalid: "%s".',
+            self::TRANSACTIONTYPE,
+            $new_color));
+        break;
+      }
+    }
+
+    return $errors;
+  }
+
 }