views.upload: Pass upload status as enum to template

Make it easier to reuse the status text messages.
views.upload: split update method

Prepare updating single flight per ajax request
macros/form.jinja: add macro to render pure fields without label
frontend.views.upload: finally redirect to a list of uploaded flights
templates/upload/result.jinja: reimplement using WTForms
Merge branch 'wtf_upload'
flights.list.jinja: remove datatables dependency
tables.css: fix table sorting header style
table_tools: fix last page of pager
flights/list.jinja: show range of current page