Only load a plugin service if its parser was used

[intjftw]

Right now, the service of every plugin is loaded for all projects anytime the webserver is started. This is redundant, e.g. if the metrics parser was not run on a project, then its service should be skipped as well.

[whisperity]

(Reimagining duplicate of #269.)

[mcserep]

(Reimagining duplicate of #269.)

Similar goal, but different purpose. The reason now is not the "1 server = 1 project" concept (which was dropped years ago), but now each plugin is loaded as many times, as many projects are contained in the workspace. Thus for performance reasons we could only load service plugins for which the corresponding parser plugin was executed.

Moving this to the next milestone.