Added eos-help-center CSS, images, and fonts

[endlessm/eos-sdk#2231]
Put Endless customizations in separate file

We will keep all modifications to the XSLT stylesheets in our own file
as much as possible.

(Roberto, Will, Fernando, Philip)

[endlessm/eos-sdk#2231]
Add divs around content

Override the html.page transform in order to provide the expected divs
around the rendered Mallard document.

(Roberto, Patrick, Will, Fernando, Philip)

[endlessm/eos-sdk#2231]
Sidebar links are in correct order and link to correct pages

[endlessm/eos-sdk#2233]
Change CSS to account for new <a> element

Adding links to the sidebar messed up the CSS, so we made a small
edit that made it look approximately as it was before.

[endlessm/eos-sdk#2233]
Translate page names on sidebar

This adds a new translation domain, 'yelp-endless', in which we
internationalize new UI strings arising from our modifications. 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]