Translate: Create a project for "Front end"

[ocean90]

Currently all the front-end strings are assigned to the main project, like "dev" or "4.3.x". This can cause confusion on pages like ​https://translate.wordpress.org/locale/de/default/wp/dev or ​https://translate.wordpress.org/locale/de/default/wp/4.3.x because it's not clear which strings the first sub-project contains.

- dev - Development
  |- front-end - Front end
  |- admin - Administration
     |- network - Network Admin 
  |- cc - Continents & Cities