CMS-3133 Input Type Textline - Adjustments when used in "multiple" mode

Fixed Dropdown position calculation.
Added `flex` model mixins.
Fixed remove button size.
Centered all elements in `input-occurrence-view` blocks, using flex
model.
Merge pull request #642 from enonic/CMS-3133

CMS-3133 Input Type Textline - Adjustments when used in "multiple" mode