Renaming a Theme allows for duplicate names

Issue Summary

Creating a Theme via the UI or API both include logic to prevent Themes with the same name being created.  However, changing the title of an existing theme doesn't include this validation and allows duplicates to be created.

This is reproducible.

Once a Theme has been renamed to have a duplicate name, it is impossible to modify it via the API because it will complain that a duplicate name exists.

Steps to Reproduce

1. Create two Themes, named "Duplicate Test 1" and "Duplicate Test 2"
2. Edit the Theme "Duplicate Test 2" and rename it to "Duplicate Test 1"

Expected Results

A popup should appear telling you a theme with that name already exists.

Actual Results

The Theme is saved with a duplicate name

Workaround

Renaming the theme to something that isn't a duplicate name works.