Allow alerts to be used with only their icon

Previously, for the alerts to work, a whitespace was needed to succeed them, and this was possible since we were using NOTE: **Note:**.

However, if we wanted to get rid of the **Note:** text, this would mean that the alert would need a trailing whitespace in order to be rendered and it would look like NOTE: .

Related issue: gitlab#290314 (closed)

With that change, the regex is more permissive and allows the whitespace to be absent.

Since now we can render alerts with only the icon present, an alternate title was also added for accessibility issues.

Needed for gitlab!44327 (merged)

Example http://support-standalone-alert-style.178.62.207.141.nip.io//ee/development/documentation/styleguide/#alert-boxes

before	after (if Note is omitted)