Merge pull request #155 from TobiasLohner/datatables_1.9.4

Upgrade to datatables 1.9.4
css: Fixed error message styling
css: Fixed warning flash message styling
form: Added Flask-WTF to the requirements
form: Extracted simple upload/form.jinja template
form: Use wtforms for upload form
form: Moved render_field() macro to separate template file
form: Set explicit action attribute for upload form
form: Use wtforms for upload results form
form: Translate 'Upload' button caption
form: Use wtforms for password change form
model: Added Model.exists() method

This can be used to query whether at least of the requested objects exists
form: Use wtforms for club change and create forms
form: Removed obsolete club forms
form: Use wtforms for club edit form
form: Simplified user form handling
form: Use wtforms for club pilot creation form
form: Use wtforms for pilot edit form
form: Use wtforms for flight pilot change form
form: Use wtforms for aircraft change form