Mark page names on sidebar for translation

This adds a new translation domain, 'yelp-endless', in which we mark
new UI strings arising from our modifications for translation. New
strings have to go in data/domains/yelp-endless.xml.in, after which the
.pot file must be updated with "make -C data/domains yelp.pot".

(Patrick, Philip)

[endlessm/eos-sdk#2233]
Run update-po

In preparation for editing the .po files and connecting to Transifex

(Patrick, Philip)

[endlessm/eos-sdk#2223]
Copy existing translations from gnome-help pages

In order to simplify translation, we have copied all existing
translations of page titles from gnome-help that were already
translated.

These translations should still be reviewed. For example, the French
translation is incomplete.

(Patrick, Philip)

[endlessm/eos-sdk#2233]