Improve bulk assignment

This MR improves current implementation of Label dropdown when used for bulk assignment on issuable pages (/:namespace/:project/issues, /:namespace/:project/merge_requests)

Previously this dropdown relied on `<input>` tags to get its active items and also to calculate items with indeterminate state.

Relying on `<input>` tags is not enough when we want to set/get multiple states on a dropdown.

For this case we want to get/set:

- Marked items 
- Unmarked items that were initially marked
- Unmarked items that were initially indeterminate
- Items with indeterminate state.

This MR makes the Label dropdown to save its own state as `data` so it will be easy to get and set whatever state we want no matter if the dropdown is filtering which is the issue that I initially wanted to solve as you can see in the following gif.

**Before** 
![2016-12-07_11.44.48](/uploads/cb697161b8b39cdee72fdbb95a531100/2016-12-07_11.44.48.gif)

**After**
![2016-12-07_11.32.43](/uploads/338255a302de0dd1367474f33232d2a3/2016-12-07_11.32.43.gif)

As you can see in the first gif the `bug` label is removed from the selected issues but the `enhancement` label should set but the `critical` should be kept. This is fixed on the next gif.

Fixes #24877

See merge request !7765