Purpose
=======

The auto-shift functionnality shifts the tasks backward or forward
thanks to two triggers :
- Manual trigger : User clicks on an arrow in the gantt view
- Auto trigger : User plans a task in conflict with a depending task
that should be shifted in order to keep a waterfall order.

This functionnality may be summarized as :
1) When a forward (resp. backward) shift is triggered, find the first
datetime in the future (resp. the past) where the assignee is available.
2) Starting from this datetime, allocate the planned hours based on
user's calendar availabilities.

Analysis
========

Currently the implementation takes a big advantage of caching the work
intervals information for a given calendar and resource and fill
efficiently the dict in order to limit the number of calls to
work_intervals_batch.

However efficient, the current implementation can be factorized in order
to have :
1) a unique method responsible of the work intervals caching which is
handled two times currently.
2) a unique method responsible of giving the new begin and end planned
dates, whatever there are or not planned hours on task
3) an efficient way to use the 'calendar validity' API of resource
introduced in PR #21135. Which allows to remove overrides in
project_enterprise_hr and project_enterprise_hr_contract.

task-2643752