Add delete button inside opened Assessment & Remove double bottom

[bubafinder]

I'm submitting a...

• [ ] Bug report
• [ ] Regression (a behavior that used to work and stopped working in a new release)
• [ ] Feature request
• [ ] Documentation issue or request
• [x] Improvement

1.

Add delete button inside opened Assessment, with corresponding confirm modal.

Right now, the assessment can only be deleted when it is closed, after clicking on the x icon.

2.

Remove double bottom on the opened Assessment element.

[hrvojevu]

@markolovric , I suggest changing delete button to mdi-delete instead of mdi-close to be more consistent and accurate.