Show crontab of users other than Apache's

[bogdanghervan]

Currently CronKeep can only interact with the crontab of the user Apache runs as (typically www-data, apache or nobody, depending on the system). Often, this is enough, but we should allow users to interact with crontabs of other users too.

On the back-end, the crontab command invocation should receive the -u argument to specify the user we are reading / writing the crontab for. We would probably need to add the user a member of the Job object, so that a job would still be easily identifiable by its hash (and keep the short URLs like /job/pause/af1d7eda).

On the front-end, we should display a dropdown somewhere. Only users with a non-empty crontab should be initially shown. The list can be expanded to show all system users by selecting the "All Users" option.

Since on most systems Apache doesn't have access to the crontab of other users, instructions on how to grant access rights should be made available as an article linked from the app itself.

Possible ways to implement the user switcher:

• display a dropdown on the right-hand side of the header;
• make the name of the current user (the one displayed in the H2 element) suggest that it can be changed and transform it to a dropdown when hovered.