ref(grouping): Expand datetime message parameterization

[lobsterkatie]

This refactors our datetime parameterization regexes to combine one set of the "old, potentially-removable" patterns (which has been spotted in the wild, so no, we can't remove it) with the current dash-optional pattern, and then expands the resulting pattern set so the colons in the time are optional as well. It then splits the set in two for greater readability. The resulting pattern set now handles the following:

• Date with or without dashes and time with or without colons

• Date and time separated either by a space or by T

• Decimal seconds, integer seconds, or no seconds

• Timezone information given as offset, as Z (UTC), or not given at all