Refactor the discovery.xml fetching

[donquixote]

We have the CoolRequest class which fetches the discovery.xml and extracts the WOPI client url from the xml.
I would like to refactor this.

Refactoring:

• Replace global functions with services.
• Avoid object state where it is not needed.

Functional changes:

• Rethink error handling.
• Don't concatenate t() strings, and rethink error message passing.
• Use Guzzle instead of file_get_contents to fetch the remote content.
• Cache the result, to avoid repeated calls. This also means we need to flush that cache at some point.
• Allow getting different values from the xml, e.g. for different mime types.

@hfiguiere some questions.
In CoolRequest we have these error codes 101, 201 etc.
Do these numbers have special meanings? Is there any external system that understands these number codes?