Event Definitions - users not permitted to create Ev. Def. can see "residual" `Create investigation` button fragment

Event Definitions - users not permitted to create Ev. Def. can see "residual" `Create investigation` button fragment

## Expected Behavior
No button visible at all.

## Current Behavior
See tiny button with arrow on the right side:

## Your Environment
https://graylog-test72.dev.torch.sh/

and local 7.2 environment

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?

Reproduce the issue in the linked Graylog test or local 7.2 environment using a user who cannot create Event Definitions. Trace how the Create investigation button is rendered and permission-checked; done means no button fragment or arrow is visible for that user.

Written by the indexing model from the issue text.