Slugable collections

Collections are now slugable. Meaning, instead of collection URL being /@mbajur/123, the number is being replaced with a parametrized version of collection name, so it becames /@mbajur/pond-news.

The logic is simple:

1. If the slug candidate is available, Collection Name is being transformed into collection-name
2. If the slug candidate is taken by any other collection on the instance, it gets transformed into collection-name-a1b2c - with random hash appended at the end of the slug.

Same collection slugs will be used as collection handles once Pond supports federation.