ANN: improvements to graphical Text editing -- links, images, and special characters

[williamstein]

We just made several improvements to the rich text editor, which is used all over in CoCalc, for chat, WYSIWYG editing of Markdown files, comments between code blocks in Jupyter notebooks, slideshow presentations, whiteboards, and more.

The + button at the top left now has several new options, including inserting links, special characters, and images:

Also, you can now single click and edit the text of a link (in edit mode), and double click (or click the link icon) to visit the destination:

Also, special character dialog, which complements emojis.