Fix Event Invites Authorization + Deleting "Special" Categories

[Watercycle]

This PR fixes the issues mentioned here: https://github.com/vkoves/carpe/pull/414#pullrequestreview-224754682. Those issues being:

1) Deleting a category on My Schedule that is marked as the default event invite category (in Edit Profile) triggers a server error.

The easy fix to this problem was removing the foreign key on the default_event_invite_category_id column. User#event_invite_category! already implements logic for dealing with how a new default_event_invite_category is chosen.

2) Random users can invite people to one another's events.

This was just an oversight since the initial event invite functionality didn't originally include proper authorization. It has been fixed by adding CanCan authorize! to all relevant controller actions.

Type of Pull Request

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

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

1) Was tested by creating a user, creating a category, setting the category as the default event invite category, and then deleting that category on the schedule.

2) Was tested by adding additional controller tests.

[Watercycle]

Note: I moved the test event host shouldn't receive an event invitation email from the EventInvitesController tests to the EventsController tests since that is the controller that defines setup_hosting_event_path.

[Watercycle]

Just to clarify, there were some old tests that I split up because they were testing multiple things at once. I only added new tests for event invites.