Automated recording

The new auto recording feature allows the storage of the talks' start times as well as end times, and uses those times to automatically start and stop recording. Auto recording also displays a countdown before and during recording, so that the speaker knows how much time there is left before a recording begins/ends. In addition, the screen flashes and there is an alert sound when the time is close to the start/end of recording.

Closes #440 

For more details, here is the link to the project proposal that addresses this issue:
https://docs.google.com/document/d/1H0pIXNEa0WHaT5iIorDOOXELvkzmSW2VEjCS1x1IwQE/edit?usp=sharing