[cms] Support syntax for translatable strings with existing default value

[kzar]

Background

Currently our CMS supports translatable strings with a syntax like {{string_name[optional comment] Default language value}}. This works great until we want to re-use the same string. We then have to duplicate the whole default language string value. (It can also lead to mistakes, if there are multiple different values specified for a string then all but one will be lost.)

What to change

Add a new shorter syntax for strings that have been used previously {{string_name}}.

• If the string _hasn't_ been used previously, and therefore a default value has not already been specified an exception should be thrown.
• Otherwise the existing default value / translation should be used.

[kzar]

​https://hg.adblockplus.org/cms/rev/8d4ad7a336cd