new extraction mode: verbatim

We want a "verbatim" extraction mode which would extract propositions (and definitions) from an existing document. There would be an interactive UI:

• Shows text of document
• suggested extractions are highlighted in-place
• highlighted sections map to anchors in the document
• displays already-linked propositions and visualises the anchors if they exist
• hover for metadata
• clicking a non-frozen suggestion allows adjustment of proposition/definition body without changing anchor