Fix #3678 and memoize user data computation

[johnjeng]

Description

Add a simple memoization method so that mentions data is only recomputed when users on the project changes or when the entire chatroom is rendered.

The latter could be removed but it turns out my store memoization method is broken. Sooo I'm going to look into that after this.

In any case, this memoization method is really easy to follow.

Also fixes #3678

Testing Steps

1. Open a project with 2 users
2. Have one user invite additional users while the other uses mentions. The list should update.
3. Check that styling hasn't regressed

Relevant Issues

#3678

Checklist:

• No debugging console.log messages.
• All new code is actually used.
• Non-obvious code has some sort of comments.

Front end:

• Restart at least one project and check its ~/.smc/local_hub/local_hub.log
• Completely restart Webpack with ./w in /src
• Completely restart the hub by killing and restarting ./start_hub.py in /src/dev/project
• Screenshots if relevant.