RBAC small improvements

[ivpanda]

1. Remove "user / service account doesn't exist" tooltip: This is because that account might exist; saying that it doesn't is confusing, and we don't need to explain the fact that it means it doesn't exist in Redpanda specifically (imo).

2. Remove the icons beside the title: On the role page and the main page, the 'refresh' buttons are next to the title. I'm not sure why these are necessary and they weren't in the design. We should remove them.

3. Correct alignment in role permissions: As shown in the screenshot, when a specific resource is named, the permissions alignment is off.

4. Correct "members" to principals: On the role page, we use 'members' instead of 'principals':

5. Filtering by name doesn't filter out roles that don't match: I've filtered here for 'test-user,' which comes up to the top, but the other users are not removed:

6. Indicate "No roles" when none are assigned where there is currently a blank space:

7. "No data found" to "no roles found" if possible in the role selection dropdown for a given user:

8. Username is automatically an error when a user is being created (as in screenshot below). Ideally, this would be just blank at the start, and only error if it's empty when the user clicks out of it.