Closes #2884 Upgrade plan link in admin bar

[remyperona]

Description

Fixes #2884

Add an upgrade plan link in the admin bar for free users with quota < 20%

Type of change

• [x] Enhancement (non-breaking change which improves an existing functionality).

Detailed scenario

• Free account with less than 20% quota remaining
• Display the imagify admin bar sub-menu
• Upgrade plan link is displayed
• Clicking on link opens the subscription popup

Technical description

Documentation

Display the button if free user and quota < 20%

Mandatory Checklist

Code validation

• [x] I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• [x] I wrote a self-explanatory code about what it does.
• [x] I protected entry points against unexpected inputs.
• [x] I did not introduce unnecessary complexity.