Web API: add "slash" parameter for routes

[pont-us]

Add an optional slash parameter to the server.api.Api.route decorator, a corresponding constructor parameter and field to ApiRoute, and code to handle this field to TornadoFramework.add_routes. Passing slash=True causes the framework to handle the specified route both with and without an additional trailing / character.

Update STAC and Coverages implementations to use this feature, which gets rid of the duplicate entries in their generated OpenAPI documentation

Fixes #965.

Checklist:

• [x] Add unit tests and/or doctests in docstrings
• [x] Add docstrings and API docs for any new/modified user-facing classes and functions
• ~[ ] New/modified features documented in docs/source/*~ n/a
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[pont-us]

@forman For some reason Codecov is ignoring this branch, but I've run coverage.py locally to confirm that test coverage has not decreased.