Merge branch 'feature/better-markdown' into 'develop'

Feature - better markdown for description

The description of the poll now has full markdown support:
- Use of ParseDown PHP library to parse markdown
- Use of SimpleMDE JS library to have an more confortable editor
- Possibility to switch from rich editor to simple textarea editor
- Possibility to configure default editor in application
- Small refactoring of adminstuds javascript

See merge request !132