Send email when reopening mailgun ticket and add tests

[rowanseymour]

No description provided.

[codecov]

Codecov Report

Merging #287 into master will increase coverage by 0.10%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   51.18%   51.29%   +0.10%     
==========================================
  Files          98       98              
  Lines        8758     8765       +7     
==========================================
+ Hits         4483     4496      +13     
+ Misses       3613     3606       -7     
- Partials      662      663       +1     

Impacted Files	Coverage Δ
services/tickets/mailgun/service.go	95.40% <71.42%> (+7.90%)	⬆️

---

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 d6c5e52...230a123. Read the comment docs.