Use item id instead of title for all data-qa-submenu-item elements

Before !129843 (merged), all nav items used the item's title (user-facing name) of the nav item for their data-qa-submenu-item attributes, used by QA specs.

With that merge request, it became clear it would be better to use more stable values for those attributes.

See !129843 (comment 1522996330) for context. Two were special-cased (project_overview and group_overview) to work around this.

A closely related problem is that it looks like only MenuItem instances have an item_id. Menu instances do not. Some Menu instances do get serialised as if they were MenuItems, though, like user profile overview, To-Do list, and so on, and end up without an id. Part of this work might be to make sure that every menu item, no matter its origin, has a unique (and stable) id.