Explicitly define structure of a valid user and database name

[theMarix]

With the current implementation validity of user and database names is left to the backend implementation. This can cause confusing issues if switched between instances using different backends and it is also easily visible to a user to see what values are valid from first principles. I suggest to explicitly define a set of valid characters, e.g. alphanumeric, dash and underscore and enforce this in the API layer instead of the backends.

[sebastianneubauer]

We can safely add "postgres" to the list of forbidden database and usernames...I heard this leads to a terrible failure of the postgraas and that is even not a surprise