[CON-171] Events Filter Improvement

[horlah]

We need to review and improve events filtering on the UI:

• Review design and implementation for calendar filter. Can it be smaller with improved UX?
• Improve dropdowns with checkbox and radio inputs. Action buttons should always be visible at the bottom of the dropdowns
• Improve design for dropdown filters, make scroll visible where needed
• Add loaders for dropdowns, communicate loading states

_CON-171