BUGFIX: couldn't upload backup on OSX

--B is not a valid df option on OSX
BUGFIX: Don't include deleted topics in activity. Deleted posts are OK.
FEATURE: better UX when backing-up the site

Users won't be prompted whenever the site goes into read-only mode.
Instead, a banner will be shown at the top of every pages.

Also, the browser won't automatically reload whenever read-only mode is
disabled. This really was bad UX.
OPTIM: change Top Topic's score computation algorithm

New algorithm will not wipe the entire table anymore and will only remove
invisible topics and add new visible topics.
Version bump to v0.9.8.8
Upgrade ember-cloaking to not trigger scroll events when the back button
is hit.
Make the back button less awkward when leaving a topic.
BUGFIX: Fix issues if the DOM element is destroyed while the interval
is ongoing (in particular this was weird for Firefox)
FIX: moderators should not be able to see site setting changes in the staff action logs. Fixes #2027
2gb ram more strongly recommended
add better new forum image
hey valid HTML is always a nice bonus
add mobile screenshots to readme
let vision stay on discourse.org
rel="prev" and rel="next" for topic also