Add posibility to add multiple plugin instances of a plugin

[coveralls]

Pull Request Test Coverage Report for Build 79

• 66 of 78 (84.62%) changed or added relevant lines in 9 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-4.5%) to 86.719%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/coffee/synyx/frontpage/DefaultPluginService.java	23	24	95.83%
src/main/java/coffee/synyx/frontpage/PluginInstanceDtoMapper.java	4	6	66.67%
src/main/java/coffee/synyx/frontpage/PluginInstanceDto.java	5	9	55.56%
src/main/java/coffee/synyx/frontpage/PluginInstance.java	9	14	64.29%
<!--	Total:	66	78	84.62%	-->

Files with Coverage Reduction	New Missed Lines	%
src/main/java/coffee/synyx/frontpage/PluginDto.java	1	75.0%
<!--	Total:	1		-->

Totals
Change from base Build 71:	-4.5%
Covered Lines:	111
Relevant Lines:	128

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💛 - Coveralls