Improved the UX of issue & milestone date picker

## What does this MR do?

Improves the UX of the date picker on issue form & milestone form.

## What are the relevant issue numbers?

Closes #18198 

## Screenshots (if relevant)

### Issues

![Screen_Shot_2016-06-06_at_09.28.29](/uploads/d9c192dff0d8076adc3ed6f005ea3790/Screen_Shot_2016-06-06_at_09.28.29.png)

### Project milestones

![Screen_Shot_2016-06-06_at_09.28.18](/uploads/2d03d72791c5fc6badfadb975a85af2b/Screen_Shot_2016-06-06_at_09.28.18.png)

### Group milestones

![Screen_Shot_2016-06-06_at_09.28.05](/uploads/0f28532396a16e6fd48ddea784ca28e3/Screen_Shot_2016-06-06_at_09.28.05.png)

See merge request !4485