[www] Code refactoring for www.go

[sndurkin]

Refactor www.go and backend.go so that functions in backend.go are smaller and more cohesive and www.go handles the incoming commands and creates the outgoing replies.

[rgeraldes]

• routing: strict slash method just needs to be applied once to the main router vs apply to all the routes (doc: any subrouters created from that route inherit the original StrictSlash setting.)
• routing: fullRoute := v1.PoliteiaWWWAPIRoute + route is not necessary as we can create a sub router based on a prefix.
• it's not easy to figure out right away the entry point of the repo: remove www.go and separate the logic between main.go (server creation and startup) and server.go
• add graceful shutdown for multiple web servers.
  (examples: https://gist.github.com/peterhellberg/38117e546c217960747aacf689af3dc2)
• Replace print + os.Exit with log.Fatal (same thing)
• replace go to in the for select with a tag before the for loop called for instance L and do a break L inside the select.
• replace *sessions.FilesystemStore with the interface sessions.Store in the PoliteiaWWW struct.
• name conventions, ex: userHttpCode should be userHTTPCode

[sndurkin]

@rgeraldes I like these cleanup ideas, although I'm not sure what you're referring to with the last one.

@marcopeereboom what do you think?

[marcopeereboom]

I think this has taken too long.