add databag menu strings to make them translatable

add menu strings to make them translatable, as databags are not fully supported by our i18n plugin (see https://github.com/numericube/lektor-i18n-plugin/issues/11).

in this way, the strings stay on the local .po files, and when they are called from the template system, they are translated 'dynamically'.

i added this contents/menu/contents.lr file to other lektors so the strings on databags will translate during build.

it contains the strings at databags/menu_footer.ini and databags/menu.ini