Mandate swap: Add new settings for mandate swap deactivation and structure level limitation

Current behavior
If a user wants to set a mandate swap, the list of possible participants is the whole participant list. This could result in the problem, that a wrong users could be selected.

Additionally mandate swap and vote delegation are oppponents to each other. Both functions are not needed at the same time. Therefore it would be good to deactivate the mandate swap.

Wanted behavior
A) Add new setting activate mandate swap under > Settings > Participants > Mandate swap [new title] > [New setting] Activate mandate swap. - Default value = deactivated.

B) In most cases the group of users for mandate swaps is limited to the structure level. Only users with the same structure level can swap mandates between each other. Taking account to this, a list with all participants is not needed.

Add a new mandate swap setting, which limit the possible swap to users in the same structure level.
The setting should be added to > Settings > Participants > Mandate swap > (setting name as sub setting to A) Activate mandate swap limitation to structure level.

Default value of it: Should not be active.

Start in Settings > Participants > Mandate swap and trace the existing mandate-swap participant selection. Add the two requested settings with their stated defaults, then verify that deactivation disables mandate swaps and structure-level limitation restricts the selectable users when enabled.