Make langindex.json more accurate

In order to solve the current chaos with incorrect strings (because of the automatic translation script), we created a new file just for mapping app strings with Moodle strings.

For example:

#  app string ---  moodle lang file / [moodle string id]

"addon.mod_workshop.weightinfo": "workshop", // It will use workshop.php file and search for weightinfo string.

"addon.mod_workshop.yourassessment": "workshop/assessmentbyyourself", // It will use workshop.php file and search for assessmentbyyourself string.

The moodle lang file would be the one in git://git.moodle.org/moodle-langpacks.git

This file is used by build_lang.sh script to create and update languages from the langpacks.

Work to do:

We should compare one by one all the strings and check if there's a better string. Then update the langindex.json file.