Create 'board_sidebar_iteration_select.vue'
to support iteration in the swimlane sidebar.

'board_sidebar_iteration_select.vue'
is a modified COPY of 'iteration_select.vue'
(used for the existing issue sidebar.)

As we widgetize the sidebar components,
(https://gitlab.com/groups/gitlab-org/-/epics/5302)
we will refactor and merge 'board_sidebar_iteration.vue'
and 'iteration_selct.vue' into a sharable, reusable widget.