Enhance anonymised nominal polls

[Elblinator]

Current behavior
A poll shows the type (nominal/non-nominal/analog) under the title

Wanted behavior
If a nominal poll was anonymised then the string should change to "nominal (anonymised)"

Button to anonymise a poll should vanish if the poll is anonymised

Reproduction:

1. Create, start and publish a nominal poll (in motions or elections)
2. Navigate into the detail view from the poll
3. In the three dot menu anonymise the poll