search: more friendly message for stale permissions

Created by: unknwon

This PR prompts a more friendly message for stale permissions when the user performs a search for the first time, which kicks off the permissions syncing process for the user.

Before:

After:

Implements RFC 109: More friendly onboarding process for user permissions

Created by: codecov[bot]

Codecov Report

Merging #8356 into master will decrease coverage by 0.14%. The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #8356      +/-   ##
==========================================
- Coverage   40.79%   40.64%   -0.15%     
==========================================
  Files        1290     1290              
  Lines       70635    68856    -1779     
  Branches     6354     6354              
==========================================
- Hits        28814    27986     -828     
+ Misses      39213    38289     -924     
+ Partials     2608     2581      -27

Impacted Files	Coverage Δ
cmd/frontend/authz/perms.go	54.54% <0%> (-5.46%)
cmd/frontend/graphqlbackend/search_alert.go	14.38% <0%> (-0.27%)
cmd/frontend/graphqlbackend/search_results.go	51.05% <0%> (-0.28%)
...d/frontend/internal/authz/bitbucketserver/store.go	68.66% <75%> (+0.11%)
cmd/gitserver/server/serverutil.go	54.07% <0%> (-4.51%)
enterprise/internal/a8n/store.go	83.28% <0%> (-4.19%)
internal/a8n/types.go	28.33% <0%> (-3.46%)
enterprise/internal/a8n/resolvers/resolver.go	39.5% <0%> (-3.46%)
enterprise/internal/a8n/service.go	34.85% <0%> (-2.93%)
enterprise/internal/a8n/resolvers/campaigns.go	49.66% <0%> (-2.34%)
... and 4 more