Delete mount actions

Goal of this MR:

• allow deletion of mount actions
• use the same delete dialog component in the whole project

Note: Currently, the deletion is only possible with the following preconditions:

• the mounted entity has no children mounted on it
• the mounted entity is not used in a dynamic location action
• the begin date and the end date of the mount action have to be equal

⚠ That means, that a user first has to set the end date to the same value as the begin date before she/he can delete a mount action!

Closes #362 (closed)
Requires backend!421 (merged)