Improve user feedback

[lukpueh]

We use Flask's message flashing for feedback from the backend (e.g. after form posts or file uploads). These flash messages are displayed as overlays and which disappear after 5 seconds.
In addition to that we show a progress bar.

Here are some suggestions to improve user feedback:

• Make user feedback contents more meaningful
• Fine tune how long error messages are shown
• Don't flash sensitive information to (c.f. Warn before uploading things that are not public keys #18)
• Mark form fields as red if they are invalid (Note: we could use Flask-WTF)
• Find better ways to encourage usage. Repeatedly flashing messages like, e.g. "And that's basically it...", and ambiguous percentages in the progress bar might frustrate the user)

[lukpueh]

• Probably it's better to not make messages disappear at all