better handling of hellthreads with mentions at bottom

src/components/mention_link/mention_link.js

src/components/mention_link/mention_link.scss

src/components/mentions_line/mentions_line.scss

src/components/rich_content/rich_content.jsx

src/components/status/status.js

src/components/status/status.vue

src/components/status_body/status_body.js

src/components/status_body/status_body.vue

src/components/status_content/status_content.js

src/components/status_content/status_content.vue