[BF] Fix admin note preview markdown. Allow some missing tags.

[listerr]

This was partly fixed in: #862

• In the admin view, the popup for the standalone view button (as opposed to the edit -> preview box) was still not rendering the MD/HTML correctly. I've made this the same as for the admin preview.
• Some of our notes contain markdown/html which was working in older versions of IXPM, but was still being stripped out.

I've allowed the following additional tags:

hr,pre,h1,h2,h3,h4,table[class],td,tr,th

This means the following markdown now works correctly:-

• # Heading

• ## Heading

• ### Heading

• ``` text blocks which insert <pre> </pre>

• --- and ---- lines which insert <hr>

• Markdown tables work.. sorta. Maybe still needs a bit of css allowing it to style properly.

Still doesn't work but probably not the end of the world:

• single backtick text `text`
• > blockquotes

Should probably update this doc https://docs.ixpmanager.org/6.4/dev/forms/

To show which html / markdown is supported,