Update menu 'Server' --> 'Servers'.
Remove server singleton customization (now can exist several).
Use Server.name as unicode representation.
Initialize defaults in API build firmware view.
Remove carriage return ('\r') to avoid problems on API representation.

* Move certificate validation to a custom model field.
* Clean certificate and pubkey form fields.
Fix request certificate action.

Update context with form that may include POSTed data.
Keep NullableTextField: backwards compatibility migrations.

Partially reverts 2e4b57cd807b77c9e4517a24b17697cf74f7d539