Add isMobile configurable

Applications can configure isMobile function, which should be called by
authorization modules whenever they need to establish if mobile vs. full
login pages need to be rendered.
Most oauth/oauth2 providers are capable of disvoring mobile
browsers without any external help. But some applications may prefer to
force a specific version (for example based on user preferences).
isMobile determination can be done on a per request basis (request is
passed as a parameter)
Extract buildAuthorizePath method

authorizePath is used to redirect user to authorization provider's login
screen
Extracting building this path into a separate configurable function
allows module developers to overwrite default everyauth behavior. It's
needed to support some quirky oauth providers that do not follow typical
patterns. It's also helpful if provider requires a different URLs for
mobile and full version of login screens.

Existing modules do not have to be changed unless (or until) they want
to use this new functionality.
tripit: display proper version of login screen

Implemented buildAuthorizeApp to redirect user either to full or mobile
version of tripit login screen
Tripit does not recognize browser type automatically
facebook: display proper type of login screen

Redirect user to full or mobile version of the application based on the
value returned by the isMobile configurable.
Older, facebook specific, functionality of forcing mobile/full version
by calling 'mobile' function is still supported.
linkedin: display mobile login screen on demand