Prevent all courier HTTP requests from accessing local networks

[rowanseymour]

Which required a bit of restructuring so that the backend which has access to the config, owns the HTTP clients used by handlers

[codecov]

Codecov Report

Merging #661 (eb08b7c) into main (8f8ef1b) will decrease coverage by 0.06%.
The diff coverage is 84.65%.

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
- Coverage   74.31%   74.25%   -0.06%     
==========================================
  Files          99       97       -2     
  Lines       13129    13134       +5     
==========================================
- Hits         9757     9753       -4     
- Misses       2684     2692       +8     
- Partials      688      689       +1     

Files	Coverage Δ
attachments.go	60.22% <100.00%> (ø)
backend.go	25.00% <ø> (ø)
handlers/africastalking/handler.go	90.42% <100.00%> (ø)
handlers/arabiacell/handler.go	75.80% <100.00%> (ø)
handlers/bandwidth/handler.go	70.77% <100.00%> (ø)
handlers/bongolive/handler.go	82.82% <100.00%> (ø)
handlers/burstsms/handler.go	84.48% <100.00%> (ø)
handlers/clickatell/handler.go	91.66% <100.00%> (ø)
handlers/clickmobile/handler.go	80.21% <100.00%> (ø)
handlers/clicksend/handler.go	77.96% <100.00%> (ø)
... and 53 more

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