Prior to this commit, some of the main styles related to event elements
in calendars were set in the Calendar module.
This created layout issues when we installed independent modules without
installing the Calendar module (eg. `planning` or `hr_payroll`).
To fix that, this commit moves the necessary styles into `web`.
task-3617329
Part of task-3575827
Part-of: odoo/odoo#154964
Brieuc-brd
2b967b59ee