Unreleased Task Icons Are Accessible to All Users

[Olda-Hal]

Icons of unreleased tasks are currently accessible to anyone, even without proper permissions. If a user visits the URL
https://rest.ksi.fi.muni.cz/taskContent/TASKID/icon/base.svg (where TASKID corresponds to an unreleased task), they can load and view the icon of that task, despite it not being released.
This behavior potentially exposes information about unreleased tasks to unauthorized users.

Steps to Reproduce

1. Identify the TASKID of a task that has not yet been released.
2. Access the following URL in a browser or through an HTTP client:
   https://rest.ksi.fi.muni.cz/taskContent/TASKID/icon/base.svg
3. Observe that the icon for the unreleased task is successfully loaded and visible.

Expected Behavior

• Icons for unreleased tasks should not be accessible to users without appropriate permissions.
• Requests to access icons of unreleased tasks should return an error (e.g., HTTP 403 Forbidden) or redirect to a placeholder image.

[xhyrom]

I can work on this one. We'll have to add authorization header in front-end (mentioned in 5ccf799#diff-bc27b2bd3c64dda24b2b33746e259a6a2c9a3bba1e80b202375bc1897524c039R162)