Switch to prosemirror-markdown for markdown textareas

[glensc]

Add markdown rendering via prosemirror to get WYSIWYM experience:

• https://prosemirror.net/examples/markdown/

Note: this is a help wanted issue, pick something from the tasks below and contribute!

---

The changes are turned off in the template. To use prosemirror, make this change first;

diff --git a/templates/include/textarea.tpl.html b/templates/include/textarea.tpl.html
@@ -1 +1 @@
-{include file="include/textarea_legacy.tpl.html"}
+{include file="include/textarea_prosemirror.tpl.html"}

Changes

• Add initial setup of prosemirror-markdown for markdown: Add prosemirror rendered markdown for textareas (disabled) #991

Blockers

• ❗ critical: all notes are submitted as "whatever was saved from garlic" so, don't use this branch for something you need to submit.
• "description" area validation fails, it thinks the field is empty. switching to WYSIWYM mode and doing some edits can workaround for now.
• the form autosave seems to be non-functioning. it always restores some older save on each note reply.
• tasklist is messed up, but check this solution: https://discuss.prosemirror.net/t/todo-list-improvements/1449/7

Nice to have

• Editor background should stay the same as textarea
• editor toolbar needs to be styled, currently white background looks weird
• Redesign of the switcher needed, it seems
• Adjust textarea minimum height, currently it jumps to the content height which on the short body is jumping area 40 rows to 10 rows. and page jumping is always bad.
• Add link and image buttons popup appears bottom of the page and has no styling. perhaps just disable the buttons for now.
• Add cmd+p to toggle views
• Add support to remember which view is in use
• the previous preview functionality? drop it? it may render differently as different renderer used.
• tabIndex wrong, shift+tab used. to switch from "textarea" to "subject" input