Replaces #8482 that was opened against master.
These problems apply to global and site-specific settings.
This pull request contains commits from master that were not merged in 3.0. Master needs to be merged in 3.0 so that this branch can be merged (rebasing this branch with the intertwined history would be way too long, too complicated, and wouldn't fix the root issue).
@mattab I have to close this one. We need to have this fix in Piwik 3 but there all this logic completely changed as they are now all part of the "Plugin Settings / Fields API". There we can eventually make a change to the API and it will be applied everywhere a textarea with multiple entries is expected.