Bug 1101192 - added labels for missing elements in Email's manual setup.

For the label IDs, we have a convention of using dash-separated word IDs, and for any new IDs we add, we like to put in a `LOCALIZATION NOTE` so that it gives the localizer some context about each item. We only started that practice after having an initial set of IDs, so that is why it is not consistent in the file yet. However, for new ones, we want to start doing it. Also, the other IDs do not have spaces around the = so it would be good to maintain that idiom for the new IDs.

It would also be good to give the ID a prefix that matches others in the same file, so in this case, `setup-manual-` and placed with those items. As an example, something like this will fit in with existing convention more:

```
# LOCALIZATION NOTE(setup-manual-security-select): For the manual setup screen, the label for security select box. This element does not have a visible label, but a label is needed for screen readers.
setup-manual-security-select.ariaLabel=Security
```

For the back button, we generally avoid general purpose labels, since many times the localization context can be different. However, in this case, the back button is fairly general, so can just go with `back-button` for it. For those general types, I suggest putting them at the top of the file, and the LOCALIZATION note should indicate is the general purpose back button with icon that uses that label.