UI/UX: Spacing

#### Curation main screen:
I decided against rearranging elements, instead I'd like to request to adjust the spacing in between elements for better content grouping - see below screenshot for the current vs mockup:
- [x] Spacing in between the "Use this image for training..." section and the "OPTIONAL" text is 30px
- [x] Spacing in between "OPTIONAL" text and "Select a base segmentation" is 20px
- [x] Increase spacing to 12px between the button row of the merging mask area and the "Excluding mask" text