Excel export of flight report

[pschuler78]

Flight reports should be made exportable as excel.

[pschuler78]

Implementation described here: https://stackoverflow.com/questions/35865887/download-excel-from-asp-net-web-api-2-using-angularjs

WebAPI POST call: /api/v1/flightreports/export/excel/{page}/{pageSize}
Post data: PageableSearchFilter (same as flight reports)

feature implemented in branch /develop