Move add customer button to the header
Rework the project/task list in the customer_form view (closes: #9a33e4)
Tweak the width of the task list table in the customer_form view
Replace the label hack in grouped select forms by a decent optgroup
Make the time_entry_form view also use a nested customer/task selector
Add specification related attributes to the Customer and Invoice model
Add the time specification to the customer_form view; update it

If time specifications are enabled for a customer this is considered to be
a global setting for invoices related to this customer.

Note that it will note change any of the existing voices, it will only
affect newly created ones!
Add the time specification to the invoice_form view; update it

Generation of a time specification appendix can be enabled for each invoice
indepently on whether it is or is not enabled for the customer.  However,
whether it is enabled by default when an invoice is created, that does
follow the setting for the customer.
Remove 12pt default for the invoice template
Generate a specification on a new page if enabled for the invoice
Tweak paragraph spacing a bit (needed in the specification part)
Add a reference to the accompanying specification if enabled on the 1st page
Merge branch 'feature/time-spec' into development (closes: #fb896d)

This implements the generation of time specifications which are seen
as an appendix in the invoice (template).  Whether this is generated
can be set globally on a customer or toggled per invoice.
Small refactoring
Add also links to time entries in the invoice_form view
Fix syntax error in case of no time specification; remove \letter call
Handle billed fixed-cost tasks correctly in the customer_form view

Billed fixed-cost tasks will have no active task once they are billed,
because invoicing a fixed-cost will not split off a task but manipulate the
active one into being billed.
Indentation fixes
Show a billed fixed-task under itself in the customer_form view

As billed fixed-cost tasks cannot have an active task, the task itself
still is the one that is billed.  Show the task under itself to expose
the fact that it is billed and in which invoice.
Fix the invoice template to show IBAN/BIC (closes: #688d33)

If IBAN/BIC is set, it is shown instead of the normal account number.
SEPA solves the problem that needs IBAN/BIC is conditionally shown (which
is not possible).