URL Design plan

[yuvipanda]

Braindump + plan from the plane!

1. Users should be able to copy paste the URL they are in and share it
   with someone else. This should "just work" - unlike how things now go with
   mybinder.org.
2. No state must be maintained anywhere.
3. Is perfectly ok to have some build redirects in there to show people proper
   UI when we need to do a build. We aren't trying to trigger rebuilds for
   requests to JS, for example.

State required for building is:

1. repo provider
2. repo slug
3. ref slug

so a URL structure could be like:

/v1/<repo-provider>/<repo-slug>/<optional-ref>/

So if we get a request for

/v1/gh/yuvipanda/environment.yml/rel/index.html

we parse this out first into:

repo_provider = gh
repo = yuvipanda/environment.yml
ref = rel
file_path = index.html

And we check if the repoprovider, repo, ref exists. If it does, we then
_manually proxy (not redirect) to the source file. This adds additional
complexity - we act as a somewhat inefficient proxy. However, that is the
only way to give the user the experience of being able to copy paste what
they see in their URL bar and 'just' have it work. That is a very important
UX affordance we must offer to people.

1. request comes in
2. We parse the parts out
3. Check if it has been built
4. If it has been built, we proxy out the appropriate files.
5. If it has not been built, we redirect to a build UI.

So simple staticfiles won't be enough.