Merge branch 'feature/hidden-and-private-polls' into 'develop'

Feature/hidden and private polls

**Added an option for the poll's hidden results**
  - At poll creation, a new checkbox is added to hide the results
  - At poll visualization, results are hidden if option is activated
  - At poll edition by the poll maker, the hidden option can be changed

**Added a new edition option**
  - At poll creation, we can choose between "no user modification" and "all users can modify every vote" like before. But we can now also select "each vote can be modified by its owner"
  - This edition option is also available for the poll maker in the poll administration interface
  - When a user votes with this new option activated, an edition link is displayed to him : only this link can allow the user to modify its vote.
  - This functionality will be useful to create "private polls" later.

**Side work**
  - The token generation is now safer, with fallback to unsafe version if the php functions are not available
  - An anchor is added to the edit link, to scroll down to the current line that the user is editing
  - Commited composer.lock (good practice)
  - Added the root htaccess.txt to keep the CLEAN_URL format updated
  - The Message class can now contain a link (only displayed in studs.php where I needed it)

See merge request !45