Merge tag 'v1.6' into development

Release 1.6
Just ignore everything under public/invoices/
Make key of customer/invoice mapping a customer

The key was the customer name before, but then it's not possible
to use any other information of the customer model anymore.
Add a "Create a new invoice" button per customer

This is done similarly as in the customer_form view.
Merge branch 'feature/50f182-new-invoice-buttons-in-invoices-tab' into development (closes: #50f182)
Add jump buttons for customer and project/task in time_entry view
Spacing and label fixes
Show time specification of projects/task the invoice_form

They are inserted in the titles of the time entry links so they are
shown when hovering over them.

Also fixed "None" not being displayed when there was no time entry
comment.
Add configuration for time resolution rounding
Add private method for the TimeEntry model to round time

The method uses the configuration to determine the resolution and is
inspired by the following StackOverflow answer:
https://stackoverflow.com/a/4182700.
Round start and end time before validation (and thus saving)

I've chosen to connect the callback to before_validation because
at a later point we want to check for overlap between time entries
and this should be done using the corrected versions.
Merge branch 'feature/9f7883-rounding-to-time-resolution' into development (closes: #9f7883)
Add some changelog entries for already finished features
Fixed bug preventing updates tasks
Documentation fix
Fix invoice period calculation

Currently, the end time of the period was always the creation time of
the invoice (due to the initialisation), as it is generally by definition
later than any of the task periods.
Check and show a delete button if an invoice file is present
Add some bottom margin to button groups
Handle delete requests for invoices
Merge branch 'feature/4235ef-force-invoice-regeneration' into development (closes: #4235ef)