Add autodelete prevention mechanism

[opatut]

There should be an option for each poll to disable auto-deletion.

• [ ] add boolean field to poll, add migration
• [ ] server-wide default value AUTODELETE_DEFAULT = False/True
• [ ] server-wide permission to edit value AUTODELETE_ALLOW_CHANGE = True/False
• [ ] autodeletion only needs to take poll value into account
• [ ] add checkbox to poll settings form, if AUTODELETE_ALLOW_CHANGE == True