Added JS style guide

Squashed. Initial version of the JS style guide.
Minor style fixes

This fixes a number of small issues described in:
https://issues.apache.org/jira/browse/COUCHDB-2370:

 - don't truncate all headers if there is space
 - fix font size of active tasks title
 - add more space to the button in config/add-section
 - fix wording after verfying the installation
 - autfocus the add-section-modal in config

Closes COUCHDB-2370
Query option tray style updates

- Query options and Api Url button and tray style updates
- New trays.less file added to group common styles across trays
- hover border now white
- Checkbox transition and effect updated
- API Url now remains in position after shrinking sidebar
- increased size of icon next to database name

Closes COUCHDB-2395
Fix hasRendered error

We are getting an error when rendering a view after removing other views
because we are deleting the old selector from the view object
Fix disappearing logout link

Also add header link when user is already logged in and is reloading
the browser which does not change the session

Closes COUCHDB-2357
Revert "Url-encode document-ids properly"

This reverts commit 92964af8ee93c53677b8abaea9463fd9ae7a4d7f.

This causes that you can not save an edited design document
Fix sidebar design

- Make height of red & grey items in sidebar equal
- Allow scrolling down to the bottom of the list when window height is small
- Better vertically aligned icons
- Also show brown padding between burger and list when scrolled down

Closes COUCHDB-2402
Fix for group_level not being explicitly passed.

Closes COUCHDB-2412
Added constants.js file to core

Closes COUCHDB-2400
Remove `rightHeader` from changes view for now

We have a separate filter function in this view. It probably
makes sense to integrate this filter into the right header
function later, but currently the rightHeader with the search for
documents and the query-options-tray does not work with changes and
duplicates the functionality of the filter in this context.

Closes COUCHDB-2397
fix bug, add regression test