Allow overriding routes

[askvortsov1]

Feature Request

Refs flarum/issue-archive#345

Is your feature request related to a problem? Please describe.
Once a route is added to FastRoute's RouteCollector, it can't really be overriden or removed. As discussed in nikic/FastRoute#111, that is not something that is planned to be changed. With #2425, a middleware could override the handler for a route, but this is hacky and not immediately intuitive.

Describe the solution you'd like
Extensions should be able to override or remove routes. While we can't do this on the FastRoute level, we actually already wrap FastRoute's route registration system in our RouteCollection class (https://github.com/flarum/core/blob/master/src/Http/RouteCollection.php). In order to allow overriding routes, we could adjust RouteCollection to only pass routes into FastRoute once all extensions have booted. addRoute would just add routes to an internal data structure, and a new method (commit()? apply()?) would actually add them to the route collector. In the meantime, if extensions register a new route with the same name as an existing one, the existing one would be overriden.

Alternatively, we could explicitly require that a route be removed, and then the replacing route be added, to avoid extensions accidentially overriding routes if they copy-paste or otherwise reuse route names.

That being said, the recommended way to insert custom logic into the execution of a controller should be listening to events, and the recommended way to redirect / wrap routes should be middleware. Replacing routes completely has its use cases, but the aforementioned approaches shouldn't be ignored in favor of just swapping the route out completely, which is more difficult for extensions to maintain.

Describe alternatives you've considered
See #2133 for some other ideas

[askvortsov1]

If core's routes are applied before extension routes (which seems sensible if we go with my proposal above), this should also fix https://github.com/flarum/core/issues/2239