Fix/settings bugs and added model SettingKeyType typing

[wolflu05]

This PR fixes various issues related to settings:

• setting get cached even there are not saved yet (have no pk) which leads to trying to recreate them which fails
  This was pretty hard to find, I located that issue with git bisect using 61481b4 as bad commit, 0ffe786 as good commit and got 16ff193 as first bad commit.
• add model key to SettingsKeyType (was actually the initiator of this PR)
• plugin slugs that contain - can't use settings because the API re_path doesn't include - chars
• related fields cannot be reseted, instead there is an js error thrown that data.value.toString() cannot be called on null
• spelling mistakes discovered while debugging in the code (I have this extension installed to prevent them while I'm coding)

[SchrodingersGat]

@wolflu05 some subtle fixes here, looks good :)