Use parseBoolean for canEdit when mounting iteration_select.vue

What does this MR do?

Fix 1

When user can't edit iteration, canEdit data attribute should be parsed using parseBoolean to prevent this error:

(parseBoolean is already used elsewhere in mount_sidebar.js)

Fix 2

We should have used .to_s to convert can_edit_issuable to string in _sidebar.html.haml. (this is only noticeable once we start parsing canEdit with parseBoolean.)

Does this MR meet the acceptance criteria?

Conformity

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