[IMP] survey: refine access mode option on survey

Purpose of this merge is to improve access mode and authentication management
introduced at 9771fdfe8f658fd0f3c270173a3d19b614ec8997 . Purpose of this cleaning is to simplify options and make
them clearer. Instead of a selection with 4 entries we propose to use two
fields on survey model :

 * access_mode: everyone with a public link can access and answer; or only
   invited people;
 * login required: whether people have to login to answer or not;

All main use cases are covered with combination of those two options.

This commit is linked to task ID 1932508 and PR #30508.

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