Package cleanup

[RasmusSkytte]

Intent

This PR makes a few, relatively minor changes to the package. Primarily focusing on clean up.
See the Approach section for the changes introduced

Approach

• The %notin% operator was removed.
  This was left over from our original migration but is not used in the package and does not really fit the scope of the package.

• The order of arguments to create_logs_if_missing() was swapped to better conform the other package functions which has conn as the first argument (unless it takes .data argument, then it takes precedence).
  create_logs_if_missing() now has conn as first argument.

• The use of %>% in examples was changed to |> to be aligned with the rest of the package.
  This also led to minor reformatting of the examples in question.

• The linting of the assertion helpers were fixed so the nolint: suppression is no longer needed

• The return values of schema_exists() is made explicit instead of implicit.

• return() was changed to return(NULL) in some places

• The triggering of db_join warnings were simplified to not rely on "testthat::is_testing" but instead use interactive() to determine if the warnings should be produced.

Known issues

This PR also contains the code being merged in

• Move functions to separate files #95

Once merged, this PR will be rebased and opened.

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR