Show device timestamps in the reader's local time

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

app/javascript/application.js

@@ -1,4 +1,5 @@
 // Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
 import "@hotwired/turbo-rails"
 import "controllers"
-import "turbo_stream_animations"
+import "turbo_stream_animations"
+import "local_time"