Occasional whiteboard lag

[jkeat]

• Browser sometimes severely slows down while using whiteboard
• Currently, we don't have specifics on reproducing
• One thing to note: reports of this happening are from volunteers with a large number of past sessions. This might be contributing to large socket.io data transmissions, which could, in turn, cause browser slow-down

[dmnisson]

I tried to reproduce this by populating my local database with over 100,000 sessions and adding them as past sessions to one of my test volunteers. I couldn't reproduce any issues with the whiteboard specifically, but I did notice that this triggered a massive slowdown when navigating between routes; this appears to result from the /api/user endpoint querying all of those past sessions from the database and calculating the volunteer session hours every time it is called. The PRs above address the issue that I found, which may be related to what some people are experiencing with the whiteboard.