use session storage instead of local storage

[ChristophWurst]

Motivation for switching to this shorter-lived storage:

• we store sensitive information in the browser storage, which lives forever if you local storage
• if the cache is old (1d old or older) chances are high it's worthless as the messages in your folders changed already and the cached ones will be deleted anyway
• cache only speeds up page refreshes, so it doesn't really help if our AJAX calls are slow
• if we built better data structures on the client side, we don't have to cache stuff. we can simply built up a data structure of accounts, folders and messages and use the data that has already been loaded before

@LukasReschke @jancborchardt @owncloud/mail please review and tell me what you think. Closing a tab or closing the browser will clear the storage, but I could not really notice the interface loading slower.

[ChristophWurst]

@jancborchardt ping

[jancborchardt]

Works, doesn’t break anything :) 👍