abbr.fish: improve support for corner cases

Handle unusual cases ('=abc', ' =abc') better - regression from
8e8e6314due to a7bab7b.

Work on #731.
abbr.fish: escape the output of abbr --show

Allows abbreviations containing embedded newlines, etc., to be displayed
and exported properly.

Work on #731.
web_config: improve abbreviations support

 * Fetch abbreviations by reading the variable directly.
 * Use space separators for writing new abbreviations.

Work on #731.