Improve top panel elements & autofit mode

[rzats]

Closes #44, closes #46:

• The autofitting feature added in Upgrade EpiVis dashboard #36 has been turned into a new navigation mode, which is enabled by default.
  • It is mutually exclusive with pan, scroll or crop modes - so once a user looks at a particular area of the chart, enabling or disabling signals will no longer reset their viewport.
  • Adding a new signal will reset the mode to "autofit" again.
• The "Scale by 1/mean" and "Reset Dataset Scaling" buttons are moved into their own group, and given more appropriate icons. Also, navigation mode icons are now positioned on the top left of the screen.
• The introductory interactive tour has been re-ordered so that top menu items are explained first.

[melange396]

This is awesome, and the new mode works well! Ive got just a few comments on presentation...

[rzats]

@melange396 updated!

[melange396]

@rzats sorry i missed this before, but we should take out this redundant "f" key (Fit) action thats been replaced by the "a" key (Autofit) action introduced in this pr: https://github.com/cmu-delphi/www-epivis/pull/54/files#diff-15c9be2e78f911b6044c296ac349b7b39aa91dc78f2c499b918f570c0990a164R52-R56