Update alerts regex to allow display of multi-line alerts
The alerts styles were introduced with !133 (diffs), but this is not perfect.
The regex is limiting us and we cannot have a multi-line alert with newlines inside it. Although there's a "workaround" for Note alert boxes (just use the blockquote style, which is the same blue as a Note but doesn't include the Note icon), there is no method for multi-line Tips, Cautions, or Dangers, which means we cannot use things like ordered and unordered lists. The workaround also doesn't include the icon for Notes.
Proposal
Use the same regex that's used in Python's rST https://python-markdown.github.io/extensions/admonition/.
Admonitions are created using the following syntax:
!!! <type> "optional explicit title within double quotes" Any number of other indented markdown elements. This is the second paragraph.
will be used as the CSS class name and as default title. It must be a single word.
Python implementation https://github.com/Python-Markdown/markdown/blob/master/markdown/extensions/admonition.py.