Bug 1727916 - Convert several customizable toolbar item names to sentence...

Bug 1727916 - Convert several customizable toolbar item names to sentence case. r=fluent-reviewers,flod,emalysz

This required migrating several strings to Fluent, and the bulk of this patch
is those migrations. The rest of the items matched up with an entry in the app
menu, so those items were switched over to use the app menu strings, and one
had to be copied in order to avoid adding a tooltip to the app menu item.

Differential Revision: https://phabricator.services.mozilla.com/D124270