Authenticate user for trip

[keiffer01]

What is a quick description of the change?

Adds an extra check on the VIEW_ACTIVITIES page to ensure that the logged-in user is a collaborator of the trip. If not, they receive an error message instead.

Is this fixing an issue?

Fixes #60

Are there more details that are relevant?

The index.js file of the ViewActivities component is added to. When the component mounts, it makes a query to Firestore to grab the collaborators for the requested trip. The possible rendered elements are:

• If the component is still fetching data form Firestore, a Loading screen is displayed.
• If Firebase returns an error, a generic error message is shown.
• If the trip doesn't exist, an error message reflecting that is shown.
• If the user is not a collaborator of the trip, an error message reflecting that is shown.
• If the user is a collaborator, the ActivityList component is rendered as normal.

Check lists (check x in [ ] of list items)

• Test written/updating
• Tests passing
• Coding style (indentation, etc)

Any additional comments?

GIF below (it is dark because I was using a Night Mode extension):