Store folders.json in app_data

[TimGeyssens]

fixes #3

[PerplexDaniel]

Thanks for your PR!

Changing the location to App_Data seems fair. However, when we merge this in and release a new version, people who upgrade will think all their folders are gone (as their file will still be in App_Plugins).

We should probably check once if there is no folders.json yet in App_Data, and if not, check if there is still one in the old location and copy it over. That way, upgrades will go smoothly as well. Looking at the code, this would go in PerplexFolder.GetRootFolder.

I can add this later, but will probably not be this week. Feel free to update this PR with it. If left like this, I will still merge it at a later point before I will add the logic described above.

[TimGeyssens]

Yeah that's right! Migrating to the new location should be included but for now we did that manually, I can confirm this does the trick on Umbraco Cloud, no more overwriting of the folders.json