Replace gl-dropdown and logic with resuable component in iteration_select and board_sidebar_iteration_select

We essentially have the same implementation across iteration_select and board_sidebar_iteration_select. While implementing bulk edit, we built iteration_dropdown which handles the fetching and dropdown template for iterations. There is some coupled logic that needs to be abstracted but should be very simple to implement in these 2 components, simplifying the component api.

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Update: #321312 (comment 526657852)

iteration_select component got converted to a widget in !54895 (merged)

The new widget replaced both iteration_select.vue and board_sidebar_iteration_select.vue

Now we need to work to make the widget work where iteration_dropdown.vue is used.