[IMP] web: add sessionStorage service

This rev. adds the 'sessionStorage' service, allowing to write and
read stuff in/from the sessionStorage (with a compatibility layer
to handle the case where the sessionStorage can't be used, e.g.
private browsing in Safari, like it is already done for the
localStorage).

This new service is necessary for the ActionManager to store the
current action, so that it can be fully restored on F5 (this comes
with the next commit).

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