Promote 'Update Gist' button to top bar

[jgierer12]

Fixes #54

To test:

1. Log in with GitHub
2. Open a Gist
3. The 'Update' button should now be visible in the top bar
4. When clicked, the button should show a loading indicator

This PR also changes the icon to Element UI's upload icon in order to better fit in with the rest of the top bar buttons. upload also matches the function of the button better than the previous icon.

[egoist]

That last missing thing is that we should only display update gist when the gist is owned by current user.

[jgierer12]

That will be possible with metadata saving in #57

[egoist]

So this is how it looks like now:

Save button is always visible, but depending on user it will have different behavior:

• not logged in: always save as anonymous gist
• logged in:
  • in /gist/:id page:
    • own this gist: update the gist
    • doesn't own this gist: create a new gist from editor
  • otherwise create a gist from editor

And there's also Save new button which is for logged in users to create new gist instead of updating gist.