Feature: add payroll user interface to employer panel

[s218491227]

This feature implements a new page into the employer panel, for Payroll

Features Implemented

• Create new page, and add it to header.js
• Include new page in app.js
• Show payroll for that specific month
• Use dropdown box to select the month to view
• Included a Guard Name filter box (not included in Figma design)
• Did not include Location in the grid, which was in the Figma design, as the current APIs don't include the location