Add `matter_full_text_uri` property to `Matter` definition and impl

While looking through the frontend and design work on the legislation tracking project, I realized for the first time that I think we may be missing a crucial piece of information which is a link to the actual full matter text.

Current `Matter` is defined as: [link](https://councildataproject.org/cdp-backend/cdp_backend.database.html#cdp_backend.database.models.Matter) but a matter definitely has full text and we should store a link to that full text. I propose `full_text_uri` or some varient of that.

Additionally, while we look into this, it would be good to investigate which `MatterFile`'s make it through the pipeline: https://github.com/CouncilDataProject/cdp-backend/blob/main/cdp_backend/pipeline/event_gather_pipeline.py#L1444

I think the above try-except block may be dropping some `MatterFile` / `MinutesItemFile` attachments that would be useful to keep and so we may want to try to fix it if we do see that behavior.