make the help button in the upper right use a tab instead of a modal

[williamstein]

0. It is currently inconsistent, since everything else is a tab.
1. This will avoid potential render inefficiency, with rerendering what is under the modal with every keystroke.
2. This fits much better with how the rest of cocalc works.
3. We can still record which file was opened when help was clicked (that will require a little extra care but it's do-able).
4. It provides an opportunity to move the Account --> Support tab to somewhere that makes sense. Info about existing support tickets should be available easily when you click "Help", not buried in account.