V8/bugfix/3498 entity service updates

[Shazwazza]

Fixes #3498

Please read notes in the tasks and related links.

The gist is:

• The EntityService for media was querying all media properties which is slow and shouldn't be done (here's a description of why it was loading in all props: EntityRepository is fetching all property data for media #3460 (comment))
• It did this in order to get the media's file path, but we can do this a nicer way by joining the umbracoMediaVersion table which contains the media path

So this is all done and tons of code is cleaned up/removed. I'd wager that nobody was ever using the code that was removed. There would be no reason why anyone was using these services and passing in a true value for loadBaseType (or full).

The _mediaFileResolvers stuff that was mentioned in one of those links still remains and that is fine, they are still used to resolve a media's path based on a full media item (i.e. when you click on a media folder in the media section... that said ... it would be much much faster and better performance to load in all media items and thumbnails when you click on a folder to use the EntityService, something to consider in the future since now it uses the MediaService which is loading in all property data!)

Testing

• Make sure that the media picker still works and shows a thumbnail
• Make sure that the media grid works and shows thumbnails when you click on a folder in the media tree that contains images
• Make sure unit tests are all passing
• review code

[bergmania]

Tested and code changes looks very good. I'll merge.

We could consider to create constants for some of the magic strings, that are used multiple times "MediaPath", "ContentTypeAlias"