Support reset of online code editor in case of merge conflicts

[krusche]

Currently, Artemis detects merge conflicts and shows an error message to the student, but cannot handle them properly, e.g. in case the online editor version contains changes that conflict with new commits in the remote repository.

One solution would be that the student gets the chance to reset the online editor version to the remote repository (or even better, but more complex: the student can choose which version should be kept, either the online editor version or the remote version)

Entry point to this issue would be https://github.com/ls1intum/ArTEMiS/blob/master/src/main/java/de/tum/in/www1/exerciseapp/service/GitService.java --> Method pull --> insert a try catch to handle this case properly

[krusche]

@thilo-behnke This is an old issue, but I think you are currently working on it anyway, so I assign it to you ;-)