Update conversion pixel to post only

[hopsoft]

Type of PR

• [ ] feature
• [x] enhancement
• [ ] bug fix
• [ ] other

Description

Update conversion pixel to POST only and support custom metadata.

Checklist

• [x] My code follows the style guidelines of this project & the tests are passing
• [x] I have performed a self-review of my own code
• [x] I have requested a review from another developer(s)

[codecov[bot]]

Codecov Report

Merging #1330 into master will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1330      +/-   ##
==========================================
+ Coverage   34.96%   34.98%   +0.01%     
==========================================
  Files         352      352              
  Lines        8114     8113       -1     
==========================================
+ Hits         2837     2838       +1     
+ Misses       5277     5275       -2     

Impacted Files	Coverage Δ
app/controllers/pixel_conversions_controller.rb	100.00% <100.00%> (ø)
app/models/pixel.rb	100.00% <100.00%> (+12.50%)	:arrow_up: