This will allow us to rely on the database to detect duplicate
`UserTopic`s (with the same `topic_name` with different cases)
and thus correctly throw IntegrityErrors when expected.
This is also important from a correctness point of view, since as
of now, when checking if topic is muted or requesting the backend for
muting a topic, the frontend does not check for case insensitivity.
There might exist duplicate UserTopics (in a case insensitive sense)
which need are removed before creating the new index.
The migration was tested manually using `./manage.py shell`.
