Nb-cron

- logging of all pre and post commands, if enabled
- updates the command that activates conda environment

Update selenium test due to deprecated find_element_by_.

Added detection of kernel for global conda environment.

Added log file on papermill command builder which pipes all commands' stdout and stderr to the specified log file.

Added parameters tag setting in notebook menu.
Use CONDA_EXE instead of CONDA_PREFIX and check if activate exists to correctly setup conda env for papermill in cron.
Added exec_start_pre and exec_start_post options to run commands before and after papermill.
Added checks to exclude empty papermill options.