fix zeplin component loading when zeplinLink is the same value for all stories

[yuqu]

Description

This PR changes the dependent variable of a hook from zeplinLink to storyId, to fix an issue where the addon cannot detect that the selected story is changed.

I also changed naming of a state variable loading to linksLoading to make things more clear. loading is also used on another state.

Why?

Users can set a global zeplinLink parameter on .storybook/preview.js file to load Zeplin components using Storybook integration feature of Zeplin. Story loading was dependent on zeplinLink and it was broken due to the fact that all components share the same link. Users could not see the corresponding Zeplin components when they had clicked on a different story on the left menu, they had to refresh the page to get it.

// .storybook/preview.js file
export const parameters = {
    zeplinLink: "https://app.zeplin.io/project/61964989324da1a90c5ec217",
    ...
};