The exact issuance and expiry times in the device rows' hover titles were
rendered in UTC, which reads as wrong to anyone not living there.
- Mark the dates with a <time> element carrying the absolute (UTC) timestamp,
rather than a custom data attribute whose name claimed to hold local time
- Rewrite those titles in the browser's own zone, keeping the server's UTC
rendering as the fallback for when JavaScript is unavailable
- Only convert values that carry an explicit offset, so a date-only <time> is
not read as UTC midnight and shifted onto the previous day
- Give the revoke button a little more room from the device name and dates
Part of #254
