plotextractor: new Plots tab
- This is part two of the merge of Valkyrie's latest plot extractor branch. (The part one was merged in 2a65ef1.) This part introduces the new Plots tab for displaying plots.
- During merge I have done slight updates to reflect new codebase status, e.g. plot context files, and done slight fixes to HTML brief format, plot detection, and Plots tab availability parts.
- Moreover, I have left several FIXMEs in the code, e.g. how to improve detection of Plots based on the doc type. We can do these at our leisure.
- The last part of Valkyrie's plot extractor branch that deals with plugging the plot extractor library into the demo site workflow will be merged later.
- This commit addresses #153.