Merge 4.21.0 into master (#3467)

.prettierignore

__tests__/__snapshots__/Storyshots.test.js.snap

android/app/build.gradle

android/app/src/main/AndroidManifest.xml

...rc/main/java/chat/rocket/reactnative/MainApplication.java

...va/chat/rocket/reactnative/generated/BasePackageList.java

android/build.gradle

android/settings.gradle

app/actions/actionsTypes.js

app/actions/rooms.js

app/constants/constantDisplayMode.js

app/containers/ActionSheet/ActionSheet.tsx

app/containers/Avatar/Avatar.tsx

app/containers/FormContainer.tsx

app/containers/TextInput.tsx

app/containers/markdown/AtMention.tsx

app/containers/markdown/Hashtag.tsx

app/containers/markdown/index.tsx

app/containers/markdown/new/BigEmoji.tsx

app/containers/markdown/new/Bold.tsx