Add UI for invoice management

Please add UI for managing invoices:

• List of clients with ability to add and edit clients
• Detail view of client, with list of invoice groups and ability to add/edit invoice groups
• Detail view of invoice group, with list of invoices, and ability to view invoice (as detail view and as PDF) and to send invoice by e-mail

mentioned in issue #2

created branch 1-add-ui-for-invoice-management to address this issue

mentioned in merge request !4 (merged)

changed the description

marked the checklist item List of clients with ability to add and edit clients as completed

marked the checklist item Detail view of client, with list of invoice groups and ability to add/edit invoice groups as completed

closed with merge request !4 (merged)

mentioned in commit 795ed859