User can edit duration of a time entry

[matijaSos]

• when duration is pressed, a pop-up with a calendar appears
• when hovered, a rounded grey background appears around duration
• when duration is pressed, an input field appears, and user can edit the text
• when duration is pressed, the whole input gets selected (blued)
• style the input's focused/unfocused states
• when the start/stop times button is clicked, open the calendar focus duration input, and select the text within it
• when the duration input field is clicked, have the same behavior as above:
  • open the datetime picker popover
  • focus the duration input field
  • select all the text within the input field
  • disable popover toggling functionality (via buttons) while the popover is open
• get rid of the explicit "Save" button in the popover - saving happens onClose of the calendar popup
• when duration is changed, validate its format
• when duration is changed, update start/stop times state vars
• when start time is changed, update duration state var
  • handle the case when start time is set to after end time (see Toggl)
• when end time is changed, update duration state var
  • handle the case when end time is set to before start time (see Toggl)