Help text styling/formatting.

For issue https://github.com/MarkUsProject/Markus/issues/1557.

Better styling for the help system's icons and popup windows, along with adding some `.html_safe` to some of the strings so the `<br>` tags work.

The `help-break` are useless now, so I deleted them. The `help-message-tooltip` class was never used, so it was also removed. The `p` tags with the actual help strings in them must be in the `span` or `div` tags for the help icon, so that they're positioned properly. (They also float over the content now, instead of shifting everything.)