Creating a "System" content type breaks Umbraco

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Creating a content type named "System" has the quite unintended side effect of breaking Umbraco entirely:

This was achieved with ModelsBuilder in PureLive (default) mode:

1. Create the content type "System" - an empty content type is fine.
2. Reload the backoffice.

This PR adds validation to prevent this specific case of unfortunate content type naming; it looks like this when you attempt to create a "System" content type:

There are likely other cases to be found down the road, and once found we can add them to the reservedModelAliases collection in the validator.

[poornimanayar]

Hey @kjac ,

Many thanks for the PR! It works as advertised. Happy to merge :-)

Poornima

[poornimanayar]

And merged!