Resolve "Permission and button to unfinish visit or to add appointment."

Closes #351 (closed)

So, visits can only be unfinished if:

• there is only one visit after the finished visit
• such future visit has no appointments
• the user has the permission to do so (or superuser)
• the visit is indeed finished

When unfinishing a visit:

• The next visit will be removed
• The visit will be marked as not finished

Visits that cannot be unfinished:

• those with more than 1 subsequent visit
• those with 1 subsequent visit with appointments set
• if the user has no permissions

Please check this comment: https://git-r3lab.uni.lu/NCER-PD/scheduling-system/-/blob/351-unfinish-visit/smash/web/models/visit.py#L50

Some tooltips and help has been added for the user.