authz: Bitbucket Server ACLs documentation

Created by: tsenart

This PR adds documentation of, and a few code changes to, the Bitbucket Server ACLs feature, including the CHANGELOG.

Test plan:

I'd like to have reviewers follow the documentation that is added in this PR.

1. Check-out this branch locally and run ./enterprise/dev/start.sh.
2. Follow the docs to setup Bitbucket Server authorization.
3. Validate that it works as expected (i.e. Sourcegraph enforces permissions defined in Bitbucket Server)

Created by: codecov[bot]

Codecov Report

Merging #4442 into master will decrease coverage by <.01%. The diff coverage is 18.51%.

@@            Coverage Diff             @@
##           master    #4442      +/-   ##
==========================================
- Coverage   47.06%   47.05%   -0.01%     
==========================================
  Files         717      717              
  Lines       43432    43453      +21     
  Branches     1741     1741              
==========================================
+ Hits        20440    20447       +7     
- Misses      21065    21080      +15     
+ Partials     1927     1926       -1

Impacted Files	Coverage Δ
pkg/extsvc/bitbucketserver/client.go	31.5% <0%> (-1.95%)
...rontend/internal/authz/bitbucketserver/provider.go	72.15% <83.33%> (+2.81%)
...d/frontend/internal/authz/authz_bitbucketserver.go	78.78% <0%> (+6.06%)

Created by: codecov[bot]

Codecov Report

Merging #4442 into master will decrease coverage by <.01%. The diff coverage is 24.13%.

@@            Coverage Diff             @@
##           master    #4442      +/-   ##
==========================================
- Coverage   47.06%   47.05%   -0.01%     
==========================================
  Files         717      717              
  Lines       43432    43455      +23     
  Branches     1741     1741              
==========================================
+ Hits        20440    20449       +9     
- Misses      21065    21080      +15     
+ Partials     1927     1926       -1

Impacted Files	Coverage Δ
pkg/extsvc/bitbucketserver/client.go	31.5% <0%> (-1.95%)
...rontend/internal/authz/bitbucketserver/provider.go	72.83% <87.5%> (+3.5%)
...d/frontend/internal/authz/authz_bitbucketserver.go	78.78% <0%> (+6.06%)

Created by: tsenart

@keegancsmith: Did you find the screenshots helpful? Everything else was clear?

Created by: keegancsmith

@keegancsmith: Did you find the screenshots helpful? Everything else was clear?

TBH I reviewed the markdown, so didn't even look at the screenshots. Which means the text was good enough! I'll take a look at the screenshots, but I am sure they make it even clearer.

Created by: tsenart

@beyang, @mrnugget: Will address your feedback post-merge, in the on-going caching branch.