feat(client): An Fx Sync relier can specify `customizeSync=true` to force the Customize Sync checkbox to be checked.

fixes #1954
Add the `importBooleanSearchParam` function to the search-param mixin.
The fx-desktop relier swallows the TypeError thrown by importBooleanSearchParam until we have startup error handling nailed down.
Merge pull request #1962 from mozilla/issue-1954-control-sync-checkbox

feat(client): An Fx Sync relier can specify `customizeSync=true` to force the Customize Sync checkbox to be checked.