Add more options in the exercise editor

[F3FFO]

Use case

After someone create an exercise it can be useful to add more options in exercise editor to ensure improvement of existing exercises

Proposal

I don't know what it can be done easily and what cannot but this is my proposal:

• edit muscles (primary and secondary)
• merge exercise togheter (in my opinion this is very important to avoid duplicate exercise like: https://wger.de/it/exercise/1097/view-base/abduktion-im-stand and https://wger.de/it/exercise/1096/view-base/abduktion-im-stand or to avoid to have the same exercise as different entity like: https://wger.de/it/exercise/924/view-base and https://wger.de/it/exercise/1129/view-base/aperturas-en-polea )
• edit variations