Fix: rapid/continuous keyboard Next event under Topics List view may lose selected item on "load more"

Using Next keyboard shortcut (J) rapidly while reaching last item in the topic list and
causing a "load more". The handler might get into a state where it can not detect any selected item due to the delay from Em.run.next
and will result in position reset.

+added boundary check of last item on next, early in the handler to avoid unnecessary computations 


https://meta.discourse.org/t/keyboard-shortcut-next-previous-continues-key-down-reset-to-first-item-in-list-on-load-more/20042
Adds govcloud region to s3 region list
FIX: Using k on Firefox to navigate up the topic list

Using k wouldn't always bring the topic in focus as you moved up the topic list
https://meta.discourse.org/t/topic-list-keyboard-shortcut-previous-k-does-not-scroll-properly-leaving-selected-item-hidden-firefox/20050
Merge pull request #2802 from cpradio/firefox-use-k-to-scroll-up

FIX: Firefox - Topic List Keyboard shortcut Previous(K) does not scroll properly leaving selected item hidden
Merge pull request #2801 from jrhe/s3_govcloud_support

Adds govcloud region to s3 region list
Merge pull request #2798 from lidlanca/patch-3

Fix: rapid/continuous keyboard Next event under Topics List view may lose selected item on "load more"
FEATURE: display reply to post on mobile
FIX: If a post has been hidden due to flagging, don't use the absolute
edit window for edit prevention.
FEATURE: fenced code blocks can hint with "pre" or "text"

Use ```pre or ```text for unhiglighted code blocks
FIX: Support ember app routing to topics with only slugs
SECURITY: Stripping links could unescape html fragments
Revert "FEATURE: fenced code blocks can hint with "pre" or "text""

This reverts commit 943ad8d1d54cc39f297198c338d484d98a4e7536.
FIX: When emoji was disabled you could get a JS error in preview
FIX: update user preferences was failing if custom_fields is blank string
Merge branch 'master' of https://github.com/discourse/discourse