fix setting of list item on configure

When setting the whole configuration via --configure, list types like
adminKey need special handling. Previously this failed as a list cannot
be appended to a list. The new code adds dedicated logic to replace the
repeated value when passing a list. Also, all items of that list are
converted into the correct (typed) form before setting them.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f