Prevent native integration & bext conflict

Created by: lguychard

Fixes #4900 (release bocker for 3.6)

The execution order of the callback passed to AJS.toInit() by the Bitbucket Server plugin and the browser extension content script can be racy, leading to conflicts between the browser extension and the Bitbucket Server native integration.

This fixes these conflicts by introducing a custom event sent by the native integration upon activation, signaling that browser extension resources should be cleaned up.

Test plan: I verified that there were no conflicts when:

• The native integration is activated before the browser extension.
• The native integration is activated later during the lifetime of the content script.

Created by: codecov[bot]

Codecov Report

Merging #4927 into master will increase coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4927      +/-   ##
==========================================
+ Coverage   48.08%   48.09%   +<.01%     
==========================================
  Files         732      732              
  Lines       44645    44645              
  Branches     1763     1763              
==========================================
+ Hits        21467    21470       +3     
+ Misses      21211    21208       -3     
  Partials     1967     1967

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/graphqlbackend.go	51.04% <0%> (+2.09%)