added end times to edit-time panel

[kylehulscher]

• when speed is changed, end time changes to reflect how long it would take to arrive when going that speed
• when start time is changed, end time is adjusted to maintain speed value
• when end time is changed, speed is adjusted to reflect the change in difference between start time and end time
• start and end time are also displayed in local time instead of utc/mgt

(new pull request because I accidentally selected main instead of dev for last one)

[vcoppe]

Hi, thank you for proposing this and sorry for the lack of feedback. I have re-implemented the website from scratch and added that to the future version. You can check it out here: https://gpx.studio/gpx.studio/ and provide feedback on this new repository: https://github.com/gpxstudio/gpx.studio.