core: expose `is_source` on `MovementHistoryListBrain`

This can be useful when making a report on movements and when we list
properties of the movements that depend on the side but are not
directly exposed on MovementHistoryListBrain. One use case was
`Movement_getSpecificReference`, which shows `source_reference` when
the brain is for the source and `destination_reference` otherwise.

With this new approach, instead of guessing we record the "is_source"
information at indexing time, when we know this for sure.

This also simplifies `MovementHistoryListBrain.date` and
`MovementHistoryListBrain.mirror_date` which no longer need to guess
the side and fix a problem that because this guessing was done using
`movement.getSourceUid()` - which cause security errors when users can
not access the source of the movement.