Add a loading indicator to My List page

[HaSistrunk]

Is your feature request related to a problem? Please describe.

When the My List page is loading, the user sees "No saved items." until the list loads. In cases where the list take some time to load, this causes confusion for the user.

Describe the solution you'd like

Display the text "Loading..." until the list is loaded, similar to our "Searching..." and "Loading..." indicators for Search Results and Collection Contents .

Describe alternatives you've considered

Similar to the Search Results page "Searching..." loading indicator, we could make the "My List" h1 say "Loading...". It is probably preferable to have the page heading visible while it loads, though, so that users know what is loading.

Additional context

A user in the Reading Room encountered this as an issue, thinking they were unable to request items when the page was taking a long time to load.