feat: can join public teams without invite

[nickoferrall]

Fix #10923

Demo: https://www.loom.com/share/43ab2b2fa5ea41b88eecfa71f7999a0f

I won't merge this until the other parts of the milestone are done.

To test

• Turn on public team feature flag for an org (see https://www.notion.so/parabol/How-to-enable-or-remove-a-feature-flag-4b4be190ed674bf8903340d6b5fa50e4)
• A user who belongs to the org can see the public teams and join them
• The teammates in the org are notified that the user has joined