Release 2019.01 feature freeze

Dear RIOTers,

We've created the 2019.01 release branch, so we are now officially in feature freeze. This means from now only bugfixes will be backported to the `2019.01-branch`.

New features can still be merged into master during this period.

The tracking issue of Release candidate 1 [1] is opened. If you want to perform some manual tests, please put your name on the corresponding items. Tasks 01 and 02 will be done automatically.

Thanks all for the awesome work!

Best regards,

Alex & Emmanuel