Allow joining public clans w/o invite token

[bukajsytlos]

[Brutus5000]

Looks good. Needs adjustment and co-release on the website before merging.

[codecov[bot]]

Codecov Report

Merging #428 (ca8e310) into develop (6c6f388) will increase coverage by 0.14%. The diff coverage is 89.58%.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #428      +/-   ##
=============================================
+ Coverage      77.00%   77.15%   +0.14%     
- Complexity      1225     1231       +6     
=============================================
  Files            249      249              
  Lines           3806     3844      +38     
  Branches         241      243       +2     
=============================================
+ Hits            2931     2966      +35     
- Misses           775      778       +3     
  Partials         100      100              

Impacted Files	Coverage Δ
...IgnoreOctetStreamToObjectHttpMessageConverter.java	28.57% <ø> (ø)
...api/config/NoopMultipartFileToStringConverter.java	66.66% <ø> (ø)
...n/java/com/faforever/api/clan/ClansController.java	66.66% <50.00%> (+2.38%)	:arrow_up:
...ava/com/faforever/api/voting/VotingController.java	9.09% <50.00%> (ø)
...rever/api/data/listeners/ClanEnricherListener.java	88.88% <75.00%> (-11.12%)	:arrow_down:
...n/java/com/faforever/api/player/PlayerService.java	71.42% <75.00%> (+4.76%)	:arrow_up:
.../main/java/com/faforever/api/clan/ClanService.java	100.00% <100.00%> (ø)
.../main/java/com/faforever/api/data/domain/Clan.java	100.00% <100.00%> (ø)
...c/main/java/com/faforever/api/error/ErrorCode.java	99.15% <100.00%> (+<0.01%)	:arrow_up:
...ain/java/com/faforever/api/map/MapsController.java	68.18% <100.00%> (ø)
... and 1 more

... and 1 file with indirect coverage changes

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