Allow showing just count for specific incident filter

[hmpf]

Bu adding "count" as a GET query prameter, return a mini-response with just the count and filter:

{
  "count": 34567,
  "params": {
    "stateful": True,
  }
}

[hmpf]

For, but not sufficient to close, #206

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@da49115). Click here to learn what that means.
The diff coverage is 50.00%.

@@            Coverage Diff            @@
##             master     #345   +/-   ##
=========================================
  Coverage          ?   72.10%           
=========================================
  Files             ?       68           
  Lines             ?     2592           
  Branches          ?        0           
=========================================
  Hits              ?     1869           
  Misses            ?      723           
  Partials          ?        0           

Impacted Files	Coverage Δ
src/argus/incident/views.py	69.58% <50.00%> (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 da49115...b48e575. Read the comment docs.