Support basepath for bitbucket server

[lkysow]

Previously we were stripping the basepath from the passed in URL. i.e.
if I passed in --bitbucket-base-url=https://me.com/basepath then we
would just strip off /basepath.

This change fixes that bug (#508).

[codecov]

Codecov Report

Merging #509 into master will decrease coverage by 1.97%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   71.48%   69.51%   -1.98%     
==========================================
  Files          62       62              
  Lines        4465     4464       -1     
==========================================
- Hits         3192     3103      -89     
- Misses       1033     1130      +97     
+ Partials      240      231       -9

Impacted Files	Coverage Δ
server/events/vcs/bitbucketserver/client.go	35.29% <100%> (+1.96%)	⬆️
server/events/runtime/apply_step_runner.go	38.18% <0%> (-48.19%)	⬇️
server/events/terraform/terraform_client.go	47.15% <0%> (-31.71%)	⬇️

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[lkysow]

Closes #508