In https://github.com/matrix-org/matrix-react-sdk/pull/4725, we changed the internal font size from 15 to 10, but the `toRem` function (currently only used for read receipts remainders curiously) was not updated. This updates the function, which restores the remainders. Fixes https://github.com/vector-im/riot-web/issues/14127
- also fiddles the font size numbers