* feat(editor): add setting to turn off spellcheck underlines
Add a device-local "Underline spelling mistakes" switch to the editing
preferences. The rich-text editor and the mobile plain-text editor now
set spellcheck from this preference and update live when it changes.
Defaults to enabled, so existing behavior is unchanged.
Refs #468
* fix(editor): keep the spellcheck setting when it cannot be saved
When localStorage rejects the write, remember the choice in memory for
the rest of the session so reopening Settings or the editor no longer
falls back to the default.
Refs #468
Dariusz Koryto
18c4833ef5