restore zipped shapefile download for vector layers in layer detail page

[boney-bun]

the value should be equal to link_name in geonode/qgis_server/signals.py

fix #504

[codecov]

Codecov Report

Merging #511 into 2.8.x-qgis_server will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           2.8.x-qgis_server     #511   +/-   ##
==================================================
  Coverage              40.92%   40.92%           
==================================================
  Files                    410      410           
  Lines                  29330    29330           
  Branches                3723     3723           
==================================================
  Hits                   12003    12003           
  Misses                 16581    16581           
  Partials                 746      746

Impacted Files	Coverage Δ
geonode/settings.py	84.96% <ø> (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 d4e9c28...c00c7ea. Read the comment docs.