Kd/add bountysource

[kdumontnu]

Add bountysource to sponsors and readme page.

Unfortunately, the "activity" badge for bountysource isn't a great metric (would be much better to have the $ amount of outstanding bounties), but unfortunately I couldn't find a way to get that. I believe this is better than nothing.

[codecov-io]

Codecov Report

Merging #14323 (ae27f34) into master (8db0372) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14323      +/-   ##
==========================================
- Coverage   41.97%   41.85%   -0.12%     
==========================================
  Files         735      744       +9     
  Lines       78933    79638     +705     
==========================================
+ Hits        33130    33335     +205     
- Misses      40347    40816     +469     
- Partials     5456     5487      +31     

Impacted Files	Coverage Δ
routers/init.go	45.79% <0.00%> (-4.61%)	⬇️
models/repo_permission.go	74.31% <0.00%> (-2.76%)	⬇️
modules/auth/auth.go	61.53% <0.00%> (-2.51%)	⬇️
modules/git/repo_commit_nogogit.go	63.33% <0.00%> (-1.67%)	⬇️
models/repo_list.go	77.87% <0.00%> (-0.89%)	⬇️
modules/auth/sso/sso.go	32.14% <0.00%> (-0.59%)	⬇️
models/user.go	53.85% <0.00%> (-0.56%)	⬇️
models/issue_comment.go	52.28% <0.00%> (-0.44%)	⬇️
routers/api/v1/admin/user.go	35.54% <0.00%> (-0.44%)	⬇️
routers/user/setting/account.go	25.12% <0.00%> (-0.39%)	⬇️
... and 47 more

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