GeezerVison

Addresses #5124

• Use layout.title.text as default download filename when neither config.toImageButtonOptions.filename or gd.fn are defined.
  • "slugify" title by lowercasing, removing HTML tags, removing most special characters, and replacing whitespace with hyphens
  • Ignore MathJax titles entirely, to avoid generating horrible filenames like ihbarfracdpsidt-v-frac-hbar22mnabla2p.png (fall back to subtitle if title is MathJax)
  • Add helper function to get title and subtitle from _fullLayout (so that template values are respected) while ignoring the "Click to add title" placeholder
• Change fallback filename to plot-image rather than newplot
• Change notifier wording for image download slightly, to better match button tooltip (remove references to "snapshot")
• Add/modify tests for the above

Steps for testing

1. Pull this branch and run devtools (npm run start)
2. Load any plot in the devtools
3. Click the "Download plot as PNG" button (camera icon) and verify that the downloaded plot filename corresponds to the plot title
4. Repeat for several more plots, including the below:
   a. scatter_marker_line_dash (normal example, filename should match)
   b. bar-like_traces_tozero (title is defined in template, filename should match)
   c. automargin-small-width (no title, should default to plot-image.png)
   d. mathjax (MathJax title, should default to plot-image.png)