Low boil 🍲: Other preliminary feature ideas

This issue is for documenting preliminary ideas that don't yet have enough detail or urgency to be made into issues.

* Help users build up a list of related work that they'll want to consult once they're finished reading
* Help users mark entities that they want to refer back to later
* “First occurrence” tag: Add a tag to results in symbol search that are the first occurrence of that symbol (or sub-symbol) in the interface.
* "Favorites" tab: Add a tab to the side bar where users can keep "favorites", or a list of references and symbol locations they want to refer back to later
* Loading indicator on paper previews: Do you see that jitter in the paper preview widget when you open up a symbol’s tooltip, or the symbol search results? Figure out how we can show a loading indicator, or better yet, a tombstone / skeleton screen (an abstract representation of a paper preview), as the preview is being rendered
* Form for processing new papers: Make a web form that users can use to request that we process new papers. Form should take an arXiv ID and a user’s email address as input, and upload the request to the database using the API.
* Table of contents / index into figures or equations