Primary key renaming

Improved consistency for existing core translation key names.

See flarum/core#265
- Completely overhauled core en.yml
- Replaced existing key names in all core JS files to match
- Extracted a hardcoded string in IndexPage.js
- Combined two app.trans calls in DiscussionControls.js
- Removed hardcoded spaces from LogInModal.js and SignUpModal.js
- Added two new keys from DiscussionControls.js (soft delete)
- Created two new “reused keys” to YML to accommodate same

js/forum/src/components/PostStream.js

@@ -221,7 +221,7 @@ class PostStream extends mixin(Component, evented) {
             if (dt > 1000 * 60 * 60 * 24 * 4) {
               content = [
                 <div className="PostStream-timeGap">
-                  <span>{app.trans('core.period_later', {period: moment.duration(dt).humanize()})}</span>
+                  <span>{app.trans('core.post_stream_time_lapsed_text', {period: moment.duration(dt).humanize()})}</span>
                 </div>,
                 content
               ];