L10n/L12y: Expose "Firefox Account" in full sentence for full localizers

[petercpg]

We have a sandwich string in

kitsune/kitsune/sumo/jinja2/handlers/403.html

Line 13 in 930dd63

{{ _('Please try to log in / sign up first with') }} <a href="{{ url('users.fxa_authentication_init') }}">{{ _('Firefox Accounts') }}</a>.

In Chinese (Taiwan) locale, the translation of the full line shall be:

請先試著使用 Firefox Accounts 登入 / 註冊
Please try using Firefox Accounts to login/sign up

Note that the first slice of sandwich is broken again into two parts in valid translation and the HTML in the center is out of localizer's control.

Usually we could move "login /register" into the last slice as a workaround in translation, but this is not possible as in this case, "Firefox Accounts" is reused in kitsune/sumo/jinja2/base.html:288 which may break other occurrences.

Changes like 32dbed0#diff-83a46d54cc7cba6ca9b1d31f9d539614 should solve such issue.

[petercpg]

Similar issue happens within this file: https://github.com/mozilla/kitsune/blob/master/kitsune/users/jinja2/users/includes/login-fxa.html , too. We should avoid these cases.