Display HH:MM for both input and display for workout duration

- input diplay is done via disable js by setting class
- add help_text to ask user for correct format
- display format is overide with customized datetime str function based on datatime Python class
Merge pull request #49 from gary-pei/master

Use HH:MM  format for both input and display on workout duration
Delete Miles for list_display (Issue #45)
Merge pull request #50 from gary-pei/master

Delete Miles for list_display (Issue #45)