Avoid recalculate of width in calendar when adding a discipline

[fjorgemota]

When adding a discipline to the list of selected disciplines, the calendar constantly readjusts the width of the columns in it.

Adjust the code to avoid that effect.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/3760718-avoid-recalculate-of-width-in-calendar-when-adding-a-discipline?utm_campaign=plugin&utm_content=tracker%2F5216072&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F5216072&utm_medium=issues&utm_source=github).