user can create multiple custom folder with same name

[ontytoom]

Current behavior:
User can create multiple custom folders with same name. They have different IDs, but there is nothing that checks uniqueness of names.

Correct behavior:
For each user, all custom folder names should be unique. (However, user A should be able to create folder folder with same name as user B, but only one such folder.)

Steps to reproduce:

1. Login to app as valid user.
2. In left menu, click Folders.
3. Click "Create a new custom folder" button.
4. For folder name enter "My first custom folder"
5. Click Create Folder.
6. In left menu, click Folders.
7. Click "Create a new custom folder" button.
8. For folder name enter "My first custom folder"
9. Click Create Folder.

Actual Result: 2 folders with exactly same name now exist.