Add header to disable bitbucket xsrf

[lkysow]

Fixes #465 which is caused by XSRF detection on Bitbucket server and
cloud.

[codecov]

Codecov Report

Merging #467 into master will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
+ Coverage   70.55%   70.57%   +0.01%     
==========================================
  Files          63       63              
  Lines        4239     4241       +2     
==========================================
+ Hits         2991     2993       +2     
  Misses       1027     1027              
  Partials      221      221

Impacted Files	Coverage Δ
server/events/vcs/bitbucketcloud/client.go	40.16% <33.33%> (+0.49%)	⬆️
server/events/vcs/bitbucketserver/client.go	30.62% <33.33%> (+0.43%)	⬆️

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