Max length for tree path

[radiac]

The max slug length of 255 seems reasonable for tag fields, but that could quickly lead to some very long paths in tag trees. If we take 2000 chars as the upper URL limit, we can set a default of 1900 to allow for most options, and document that the start of a path URL can only safely be 100 chars long.

The main problem with this is that paths are auto-generated, so we have to manage this automatically. I suggest that once a path gets to eg 500 characters its no longer human-readable anyway, so instead of using tag slugs in the path, we use tag ids (for the children of the first tag to break the 500 char limit).

These limits will be configurable as site-wide settings. Upgrade instructions will need to warn of the change.