Follow Up to supporting attachment import from GHE Server

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A few follow-up tasks from Add support for ghe domains (!197673 - merged) need to be done:

• Do not update file/image attachment URLs in code/DiffNotes (see !197673 (comment 2690596694)+)
• update description in specs. (see !197673 (comment 2690354663)+ !197673 (comment 2690355179)+
• fix rubocop offence and throw an error (see !197673 (comment 2690596662)+)
• add method enterprise_import? on client class (see !197673 (comment 2690596667)+)
• update comment to include client arg (see !197673 (comment 2690596673)+ and !197673 (comment 2690596680)+)
• always initialize a MarkdownText object (see !197673 (comment 2690596681)+ and !197673 (comment 2690596683)+)
• move web_endpoint sanitizing logic to client (see !197673 (comment 2690596678)+)