Add regex validation to entity model

What does this MR do and why?

This change adds regex validation to destination_name (aka destination_slug), destination_namespace, and source_full_path attributes of a BulkImports::Entity model. This validation ensures respective values are in the accepted format.

The validation uses the Gitlab::Regex module, and I've updated/renamed/added regexes in the module to cover the different formats of a destination_namespace (which can be an empty string in the case of an api POST request) and a source_full_path, which cannot be an empty string.

While there are a large number of changes in the MR, the bulk of these are simply updating specs to use correctly formatted values for these attributes (e.g. changing 'My Destination Path' to 'My-Destination-Path').

It completes work started in #383760 (closed), which added similar validations to the GraphQL api endpoint.

MR acceptance checklist

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Related to #388385 (closed)