html: Avoid corrupting quoted style attributes

For instance, current this style we be converted as
```
<span style="font-family:'courier new';">
```
to
```
<span style="font-family:&quot;">
```

Also discard Microsoft Office specific style attributes such as `mso-list`
and friends
html: Trim space around `;` in @style attributes
Merge pull request #784 from greezybacon/issue/html-style-cleanup

html: Avoid corrupting quoted style attributes

Reviewed-By: Peter Rotich <peter@osticket.com>