Merge branch 'master' into glitch-soc/tentative-merge

Conflicts:
	README.md
	app/controllers/statuses_controller.rb
	app/lib/feed_manager.rb
	config/navigation.rb
	spec/lib/feed_manager_spec.rb

Conflicts were resolved by taking both versions for each change.
This means the two filter systems (glitch-soc's keyword mutes and tootsuite's
custom filters) are in place, which will be changed in a follow-up commit.

Dockerfile

@@ -6,8 +6,10 @@ LABEL maintainer="https://github.com/tootsuite/mastodon" \
 ARG UID=991
 ARG GID=991
 
-ENV RAILS_SERVE_STATIC_FILES=true \
-    RAILS_ENV=production NODE_ENV=production
+ENV PATH=/mastodon/bin:$PATH \
+    RAILS_SERVE_STATIC_FILES=true \
+    RAILS_ENV=production \
+    NODE_ENV=production
 
 ARG YARN_VERSION=1.3.2
 ARG YARN_DOWNLOAD_SHA256=6cfe82e530ef0837212f13e45c1565ba53f5199eec2527b85ecbcd88bf26821d
@@ -50,6 +52,7 @@ RUN apk -U upgrade \
  && rm yarn.tar.gz \
  && mv /tmp/src/yarn-v$YARN_VERSION /opt/yarn \
  && ln -s /opt/yarn/bin/yarn /usr/local/bin/yarn \
+ && ln -s /opt/yarn/bin/yarnpkg /usr/local/bin/yarnpkg \
  && wget -O libiconv.tar.gz "https://ftp.gnu.org/pub/gnu/libiconv/libiconv-$LIBICONV_VERSION.tar.gz" \
  && echo "$LIBICONV_DOWNLOAD_SHA256 *libiconv.tar.gz" | sha256sum -c - \
  && tar -xzf libiconv.tar.gz -C /tmp/src \