add registration form for managed server service

[sualko]

cc @MarcelWaldvogel

Tasks:

• replace registration url
• add bosh url to csp

[codecov]

Codecov Report

Merging #37 into master will decrease coverage by 0.39%.
The diff coverage is 62.92%.

@@            Coverage Diff             @@
##             master     #37     +/-   ##
==========================================
- Coverage        70%   69.6%   -0.4%     
- Complexity      229     250     +21     
==========================================
  Files            27      29      +2     
  Lines           800     885     +85     
==========================================
+ Hits            560     616     +56     
- Misses          240     269     +29

Impacted Files	Coverage Δ	Complexity Δ
lib/DataRetriever.php	0% <0%> (ø)	6 <6> (?)
lib/Settings/Admin.php	0% <0%> (ø)	5 <0> (ø)	⬇️
lib/Controller/ExternalApiController.php	78.08% <0%> (ø)	22 <0> (ø)	⬇️
lib/Controller/ManagedServerController.php	100% <100%> (ø)	15 <15> (?)

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