Improve best time to meet algorithm

[taneliang]

• Integrate best time to meet algo into UI.
• Ensure that slots with 0 people aren't considered. This can be done by giving them a very low negative value.
• Implement key people filter. This can be done by giving these people very high weights.
• Check that a meeting has a constant set of attendees. The current max range algorithm tries to find the meeting time where the highest number of people can make it for the longest duration. However, as we only consider the number of people and not the actual people, the set of people who can attend the meeting may change throughout the meeting. This may be intentional - it may be okay for people to leave and arrive partway through the meeting, but it should be a setting available to the user.