Leverage islandora_events if it exists

[joecorall]

GitHub Issue: Islandora/documentation#2395

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[adam-vessey]

The source_mimetype stuff is probably fine; however otherwise, there seems to be a weird smell around things.

I might be misreading, but everything else here could be done without modifying the code of this module? As in,

• hook_module_implements_alter() could be used to suppress the various hook implementations that islandora_events intends to replace
• Overriding the islandora.eventgenerator class from another module could allow the service's behavior to be overridden in whatever way that is desired

Might also be served by overriding the islandora.utils class to take over the various execute*Reactions() methods?

[joecorall]

Thanks - yeah this PR is just a hack right now so I can test the stuff locally.