use absolute server url in notification creation

[karakayasemi]

• Fixes https://github.com/owncloud/announcementcenter/issues/89
• Fixes https://github.com/owncloud/enterprise/issues/3579

[codecov[bot]]

Codecov Report

Merging #136 into master will not change coverage. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #136   +/-   ##
=========================================
  Coverage     99.23%   99.23%           
  Complexity       63       63           
=========================================
  Files             8        8           
  Lines           260      260           
=========================================
  Hits            258      258           
  Misses            2        2

Impacted Files	Coverage Δ	Complexity Δ
lib/BackgroundJob.php	100% <100%> (ø)	7 <0> (ø)	:arrow_down:

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