Event Invites Miscellaneous

[Watercycle]

This PR adds several small things:

• Allows you to add and remove invited users in events (Resolves #396)
• Fixes a bug from #378 that would break empty schedules
• Fixes the drop-down z-index in the groups user inviter (Resolves #398)
• Maybe fixes the delay while sending event invites (Resolves #397)
• Added the privacy restrictions from #370 (Resolves #374)

It's still not all that pretty, but is the visual change.

Type of Pull Request

Based on the [contributor's guide][contrib-guide], this PR is of type:

• [x] Development (feature-branch -> dev)
• [ ] Hotfix (hotfix-branch -> master)
• [ ] Release (release-branch -> master)
• [ ] Other

Requestor Checklist

Requestor: Put an x in all that apply. You can check boxes after the PR has been made.

Reviewer: If you see an item that is not checked that you believe should be, comment on that as part of your review.

• [ ] Code Quality: I have written tests to ensure that my changes work and handle edge cases
• [x] Code Quality: I have documented my changes thoroughly (using [JSDoc][jsdoc] in Javascript)
• [x] Process: I have linked relevant issues, [marking issues][gh-marking-issues] that this PR resolves
• [x] Process: I have requested at least as many reviews required for this PR type (2 or 3)
• [x] Process: I have added this PR to the relevant quarterly milestone
• [x] Process: I have tested this PR locally and verified it does what it should

How This Has Been Tested

Unit tests and testing the individual things addressed.

[vkoves]

@Watercycle - can you fill out the requester checklist?

[Watercycle]

@vkoves Certainly. Let me know if anything in particular is missing / needs to looked at more closely.

[Watercycle]

CI is failing. I'm not sure why yet.

Selenium::WebDriver::Error::WebDriverError: chrome not reachable

headless chrome: 73.0.3683.75
chromedriver: 2.46.628388 (supports chrome v71-73)
selenium-webdriver: 3.141.0

[Watercycle]

CI addressed in #403.