Replace JavaScript Testing Framework from Mocha to Jest (#5412)

.eslintrc.yml

.travis.yml

...on/components/__tests__/__snapshots__/avatar-test.js.snap

...nents/__tests__/__snapshots__/avatar_overlay-test.js.snap

...on/components/__tests__/__snapshots__/button-test.js.snap

...ponents/__tests__/__snapshots__/display_name-test.js.snap

app/javascript/mastodon/components/__tests__/avatar-test.js

...ript/mastodon/components/__tests__/avatar_overlay-test.js

app/javascript/mastodon/components/__tests__/button-test.js

...script/mastodon/components/__tests__/display_name-test.js

...avascript/mastodon/features/emoji/__tests__/emoji-test.js

...ipt/mastodon/features/emoji/__tests__/emoji_index-test.js

.../mastodon/features/ui/components/__tests__/column-test.js

app/javascript/mastodon/test_setup.js

jest.config.js

package.json

spec/javascript/.eslintrc.yml

spec/javascript/components/avatar.test.js

spec/javascript/components/button.test.js

spec/javascript/components/emoji_index.test.js