[cms] Allow to specify default translation inline in pages rather than in a separate file

[trev]

Background

Currently, we require all translations to be inside .json files. While this makes uploading translations to Crowdin easier, it also means that editing page files is very complicated - one has to constantly match string references to the strings in the locale file.

What to change

Allow specifying the default translation (typically English) inline in the page. The format should be as follows:

Raw HTML:

<p>{{click_text Click <a href="data:">here</a>.}}

<p>{{click_text[Optional comment for translators] Click <a href="data:">here</a>.}}

Markdown:

<p>{{click_text Click [here](data:).}}

<p>{{click_text[Optional comment for translators] Click [here](data:).}}

Jinja2 template:

<p>{{'Click <a href="data:">here</a>.'|translate("click_text")}}

<p>{{'Click <a href="data:">here</a>.'|translate("click_text", comment="Optional comment for translators")}}

In some cases, Jinja2 templates will still need to read data from a locale file. No default will be specified then, meaning that a .json file is still required for English here:

<p>{{get_string("toc_title", "interface")}}</p>

Note that there is a conceptual change here: currently, the CMS looks at the locale files whenever it needs to determine which locales are available for a page. Now it has to assume that each page is automatically available in the default locale, regardless of whether a corresponding locale file exists.

[trev]

Fixed: ​https://hg.adblockplus.org/cms/rev/b022896ef69a

[Ross]

Implemented and working.

Tested by:
Creating a CMS page and creating various translatable links (HTML, Markdown, Jinja). Also running the website converter script as per the read me and checking the output.

Tested with:
www / r18024
sitescripts / r501
web.adblockplus.org / r68
website-converter / master / 105208883c4336a3788820fe9ab379cf639f8387