Rewire frontend notifications to backend notifications API

[knolleary]

Closes #4245

This is the 3rd of three PRs to introduce the new notifications system

1. Initial user notification backend #4164
2. Add notification when user team invite is created #4183
3. Rewire frontend notifications to backend notifications API #4254 (THIS PR)

Description

This updates the notification frontend to use the new notifications backend API.

Summary of changes:

• The account store handling of notifications has been simplified; it now just stores the list of notifications from the api without separating out by type.
• The account store now handles the user invitations as a separate property
• The various notification counts now pull from the appropriate places; either the unread notification count (for the top-right notification button), or the invitation count (for the places specifically concerned with invitations
• The notifications have a visually distinct read and unread state (see below).
• Clicking on a notification will mark it as read
• Work done in the underlying branches now means a team-invite type notification will get deleted when the invite is actioned.
• Fixes a couple vue warnings present in the current code around missing required properties on the Notification component.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.13%. Comparing base (fbcca84) to head (8cd53a3).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           4165-team-invite-notification    #4254   +/-   ##
==============================================================
  Coverage                          78.13%   78.13%           
==============================================================
  Files                                292      292           
  Lines                              13403    13403           
  Branches                            3000     3000           
==============================================================
  Hits                               10473    10473           
  Misses                              2930     2930           

Flag	Coverage Δ
backend	78.13% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[joepavitt]

Approved, although not merging as to not impact other PRs for the upstream branches

[knolleary]

Thanks @joepavitt - there's one PR left to review, which I'll get done on Monday and then we can merge the whole stack of them.