Narrow list of tags to related ones

When a tag is selected, the tag list narrows to show only the related
ones: all tags associated to the currently visible notes. Clicking on
the plus sign near another tag narrows the list again to the tags of
notes associated with the firstly AND secondly selected tag. To show
every tags again, press the tag icon on the top-left corner of
Boostnote.

Before:
![screencast](https://i.imgur.com/PwAdhLe.gif)

After:
![screencast](https://i.imgur.com/s3JCaFq.gif)

NOTE: Tags are joined with `&` character (`#` not works) in
`location.pathname` thus it will make the tags with this character
unavailable. Any suggestion to pass multiple values via pathname?

browser/main/index.js

@@ -112,6 +112,9 @@ ReactDOM.render((
           <IndexRedirect to='/alltags' />
           <Route path=':tagname' />
         </Route>
+        <Route path='narrowToTag'>
+          <Route path=':tag' />
+        </Route>
         <Route path='storages'>
           <IndexRedirect to='/home' />
           <Route path=':storageKey'>