Enable nightly CI jobs

[urfeex]

For now I've only added the standalone builds and tests.

This should give us more insight on flaky tests. Also, this runs the URL check so we find broken links earlier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.82%. Comparing base (316b1b3) to head (b6467c4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   72.62%   72.82%   +0.20%     
==========================================
  Files          76       76              
  Lines        2977     2970       -7     
  Branches      381      381              
==========================================
+ Hits         2162     2163       +1     
+ Misses        614      606       -8     
  Partials      201      201              

Flag	Coverage Δ
ur20-latest	71.48% <ø> (ø)
ur5-3.14.3	72.37% <ø> (+0.17%)	⬆️
ur5e-5.9.4	72.11% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.