csv: using caching to create birthday list.
csv: handling Feb-29 case when exporting birthday list.
csv: adding bv to birthday table.
csv: caching User#bv
csv: using cached bv in birthday list.

this only a temporary solution. we'll remove the bv from the list again.
csv: revert csv column in birthday list.

this has to be in wingolfsplattform rather than your_platform.
new users (Aktivmeldung): assign user to the first status group of a corporation rather than to just the group called 'Hospitanten'. There are corporations that call that group differently.
fix: new users: assign the user also if the corporation_id is a string, which is the case if given by a html form.
csv: adding byte order mark to make it compatible to ms excel.
list export: adding the 'to_xls' gem.

This gem will handle export to native xls files, based on the
‚spreadsheet‘ gem.
adding basic ListExport class.

This will handle CSV and XLS exports and possibly others.

Trello: https://trello.com/c/xYrNDwgv/581-csv-listen-export
list export: Implementing birthday list export.

I’ve used the existing one `app/models/group_mixins/csv` as a template.

The new one is implemented in the ListExport class.

After migrating all presets, the controller has to be adapted to use
the ListExport class and the old specs and csv additions have to be
removed.
list export: migrating the member_names list.
list export: migrating address_list.
list export: migrating the phone_list.
list export: migrating email_list.
list export: migrating member_development list.
list export: some clean-up.
list export: switching groups#show for CSV and XLS to the new ListExport mechanism.
list export: including the xls tables in the download menu of groups#show.