Allow plain usernames without domain for sharedRoster

[MarcelWaldvogel]

Similar to auth() and isuser()

[codecov]

Codecov Report

Merging #54 into master will increase coverage by 8.18%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master      #54      +/-   ##
============================================
+ Coverage     69.77%   77.95%   +8.18%     
- Complexity      250      261      +11     
============================================
  Files            29       29              
  Lines           890      921      +31     
============================================
+ Hits            621      718      +97     
+ Misses          269      203      -66

Impacted Files	Coverage Δ	Complexity Δ
lib/Controller/ExternalApiController.php	77.33% <85.71%> (-0.75%)	24 <0> (+2)
lib/stanzahandlers/iq.php	100% <0%> (ø)	6% <0%> (+1%)	⬆️
lib/command/refreshroster.php
lib/Settings/Personal.php	0% <0%> (ø)	8% <0%> (?)
lib/db/presencemapper.php	100% <0%> (+5.97%)	15% <0%> (ø)	⬇️
lib/db/stanzamapper.php	100% <0%> (+6.89%)	7% <0%> (ø)	⬇️
lib/db/iqrosterpush.php	100% <0%> (+100%)	1% <0%> (ø)	⬇️
lib/hooks.php	100% <0%> (+100%)	9% <0%> (ø)	⬇️
lib/rosterpush.php	100% <0%> (+100%)	11% <0%> (+4%)	⬆️

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