FE 1.5 - Make the table cells editable

[LeandroHamaguchi]

Make the table cells editable in the Manage Staff page and in the Volunteers table page.

• Editing a cell should also edit the respective record in the database.
• Use the existing backend API functions to update the database.
• When a user starts editing a cell, there should be a Save and a Cancel button show up at the bottom right of the page. This means the user can edit multiple cells before pressing Save to update the database with the edited data. If the user presses Cancel, all modifications to the edited cells are reverted.
• If FE 1.0 - Add button to remove volunteers #115 has already been implemented, make sure that this feature is compatible with it.