Fix survey export: Remove Excel in favor of Exportables

[Maximilian-Staab]

The exporting of survey results is currently using a no longer supported call to Excel::create. Both AttendeesExport and OrdersExport already exist and are used by their respective controller.

This fix implements the same functionality for surveys.

Should fix #1015

This is my first time using php in any meaningful capacity, please be gentle.

[ghoussay]

I have applied your changes but got a timeout by php (maximum execution time...)
I checked your query and it appears it combines way too much lines. (result arround 600000 lignes for 7 attendees and 6 questions).
I changed the query like this (should join attendees table on attendee id) :

    public function query()
    {
        $query = QuestionAnswer::query()->where('question_answers.event_id', $this->event_id);
        $query->select([
            'attendees.first_name',
            'attendees.last_name',
            'attendees.email',
            'questions.title',
            'question_answers.answer_text',
        ])
            ->join('attendees', 'attendees.id', '=', 'question_answers.attendee_id')
            ->join('questions', 'questions.id', '=', 'question_answers.question_id');
        return $query;
    }

[justynpride]

Perfect, I needed this. Now to get the questions editable and viewable on the attendee record and we have a user friendly system

[gaforester]

I updated to 2.8 thinking this might be fixed. Nope. Not sure why this was never merged but it works great. I just changed the attendees join line to ->join('attendees', 'attendees.id', '=', 'question_answers.attendee_id') and it works perfect. Hopefully this can get pushed to a future release.