Frontend for epic start date and end date

• Sidebar in the epic to include start date and end date.
• There is no way for the user to select invalid dates in the date picker (similar to due date in issues): start date must be less than or equal to end date (Can be same day).
• Both fields are optional.
• Permissions for changing the fields are the same as the permissions for editing the title/description of the epic, as described in https://gitlab.com/gitlab-org/gitlab-ee/issues/3550.
• Ability to expand/collapse sidebar (similar to other sidebars)
  • When collapsed, the behavior and information shown should be the same as milestone dates

Expanded	Collapsed: empty	Collapsed: start only	Collapsed: finish only	Collapsed: start and finish