Tag slugs can be forced to uppercase and made similar to existing ones

[emilghittasv]

Preconditions
Sign in to SUMO with an account that has admin permissions.

Steps to reproduce
Steps to reproduce the behavior:

1. Go to /admin/taggit/tag/add/
2. Add a tag which is similar to an existing one but force the slug to be in uppercase. Example create a test topic with slug Test (there is an existing test topic with slug test
3. Access a random forum question and try adding the test topic,

Expected behavior
Uppercase slugs similar to existing slugs cannot be added.

Actual behavior
Uppercase slugs which are similar to existing ones can be added via admin and trying to apply them to questions returns Tag.MultipleObjectsReturned https://mozilla.sentry.io/issues/6047040063/?project=4504326797197312&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&statsPeriod=24h&stream_index=0

Screencasts