render 410 page when link has been used/expired

[goenning]

Issue: closes #170

Introduces the 410 GONE page used for verification keys that have been user or expired

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f70ecbd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #335   +/-   ##
=========================================
  Coverage          ?   72.84%           
=========================================
  Files             ?       71           
  Lines             ?     4511           
  Branches          ?        0           
=========================================
  Hits              ?     3286           
  Misses            ?      982           
  Partials          ?      243

Impacted Files	Coverage Δ
app/pkg/web/context.go	80.09% <100%> (ø)
app/pkg/web/renderer.go	75.38% <100%> (ø)
app/handlers/common.go	84.85% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f70ecbd...fa21564. Read the comment docs.