brandname-812762: Remove 'Firefox' (attempt 2)

Partly we shouldn't have this string in the web version anyway, but also
we shouldn't even have it in Firefox due to other brandings like Aurora
IceWeasel, etc.

See previous commits on this branch for history.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
introtext2-848333: Update forgotten comments

When we updated the introTextOpening in bug 812762, we missed some
references in other comments. This patch updates those changes.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
landjetpack-793928: Move promise library in moz-central

Catchup with change to mozilla-central: Switch users of the promise
library to the new location and move tests files.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
introtext2-848333: Fix missing l10n string

Unclear how this introTextKeys change got left behind, but it needs
updating.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
Merge branch 'introtext2-848333'
Merge branch 'master' into asynctypes-685526