* Add Playground struct and basic support for creating it and retrieving
it
* Add missing functions in pwd mock
* Get playground from request domain and validate it exists. If valid set
it on the newly created session.
* Move playground specific configurations to the playground struct and use
it everytime we need that conf.
* Don't allow to specify a duration bigger that the allowed in the
playground
* Add support for setting alias when creatign instance.
The POST to create a instance now provides an `alias` field which
then can be used to access the instance services through the following
URL:
`http://<alias>-<short_session>-<port>.<tld>`
When creating a session you can now send an `alias`
* Remove unnecessary function
* Add alias support for DNS resolution
It fixes lots of bugs, can fallback to long polling, resize viewport of
terminals and share clients state of the session, so they all see the
same thing.
