Review which data fields we're actually using

[tpoliaw]

The model structs we currently have closely follow the data sent by tiled but much of it is not used or not reliable.

• Search responses have a "meta" field with optional and arbitrary data
• RunMetadata has a "start: Start" field mapping to the start document but this is never used in glazed (and can't be relied on to be anything more than an arbitrary json value).
• Attributes has "access_blob", "specs",
• Links contains a combination of all the link fields used by any of the types in tiled and might be better served by a hashmap
• etc

It might be useful to remove the fields we're not using and replacing them if and when they're being used in glazed.

It would make testing a lot easier when we don't have to return a vast amount of unrelated data from mock servers to test query handling.