Summarize citations in all files in a project

[krassowski]

When writing up a paper I want to know how many times I cited specific piece in my notebooks, and in which ones. This could be implemented as a server extension that walks the project directory and show the summary of citations. Things to consider:

• should show progress as it can take a bit of time (already got a widget for that)
• should use notebook metadata to obtain citations; should count items/DOI rather than cluster ids
• should allow to limit search to a specific directory
• should allow clicking on the notebook with citation to open it
• (optional) should allow to filter out files from .gitignore

UI: a button in the sidebar as a trigger; results will be displayed in a separate main area widget.