add webapp deploy tokens

(note this PR is made over !203 (merged), you should merge it first to ease the review)

Add webapp deploy tokens:

• add the Token model to store the authentication tokens
• add the WebappTokenCreate & WebappTokenDelete views to manage the tokens
• enable the docker panel in the WebappSandboxPanel
• rename the jwt token class 'Token' as 'JwtToken' (to avoid any confusion)
• enable access to the registry (only for pushing for the moment)

Other changes:

• use token instead of token for the username in token auth (because escaping '' in the shell is an unnecessary burden)
• allow setting the WebappVersion.description when pushing an image with docker
• make the push/pull error messages compatible with the docker registry api