We silently upgrade existing abbreviations and change the separator when
saving.
This does not yet warn when the user is using the old syntax.
Resolves#2051
Support for space-delimited abbreviations was added to the expansion
parser in fbade198; this commit extends that support to the user-facing
tools, and documents the space-separated behaviour. Equals-delimited
abbreviations are expected to be removed before the next release.
Work on #731.