Implemented Invoice Manager

Related Issue

Closes #74

New invoice manager view (navigate to it using the view links on the left side navbar).
Invoices can be created by filling out the input fields and clicking the save button. Input fields must be filled out correctly.
Invoices can be updated by clicking on an existing invoice in the grid table and updating the input fields then clicking save.
Invoices can be deleted by clicking on an existing invoice in the grid table and clicking the delete button.
Invoice details are saved for each user between sessions. Data is persisted and each invoice has a UUID generated for DB management.

[x] Can you navigate to the invoice management view?
[x] Can you successfully create an invoice?
[x] Can you successfully update an invoice?
[x] Can you successfully delete an invoice?
[x] Does the data persist? (i.e. Do the already created invoices display after leaving the page, reloading, etc?)

N/A