Remove duplicate button is invisible to non-admins

This button:
https://github.com/google/clusterfuzz/blob/cbb244ca005b77e0e9432f74d5b55311e8827973/src/appengine/private/components/testcase-detail/testcase-detail.html#L429

is invisible to non-"admins" so it appears the functionality does not exist.

- It should be visible but disabled instead
- The text should describe it as an admin function for BOTH admins and non-admins so it's easier to communicate about
- It might be useful if it were _not_ an admin function, because handling false dedups (in my case [caused by a common flake](https://crbug.com/534112967)) doesn't seem like it should be the responsibility of admins only. However I'm not sure whether I can achieve the same thing just through a "redo" - if that will unmark it as a duplicate once the flake goes away. If that's the case then this button should explain that as well.

Start in src/appengine/private/components/testcase-detail/testcase-detail.html at the linked button around line 429, and trace the condition that hides it from non-admins. Confirm the expected behavior for disabled visibility, shared admin wording, and whether redo changes duplicate status; done means the behavior and explanatory text are clear for both roles.

Written by the indexing model from the issue text.