Improve UI for logging offline sessions

From feedback following came up:

• The position: The form is on the profile page. One user expected this feature to be at the Meditation screen.
• Instead of "date" / "time" make clear it is the starting time of the session
• For "walking" / "sitting" better indicate it's the amount of minutes.
• Sometimes user might want to add sessions for multiple days at once. Separate Issue: #277 (closed)