Add a link to a page to setup the Hypothesis client in navbar

### Problem you are trying to address with this feature

There are no links on the new site to a page which tells users how to install the Hypothesis client, in case they are either new users who don't have the tool yet or existing users who don't have it set up in the browser they are currently using.

### Your solution

Add an "Install Hypothesis" or "Get Hypothesis" link to the settings dropdown menu which links to a page with information on how to install the client.

To really address the needs of new users who land on the search page, we'd have to do a lot more - since they won't know what Hypothesis is, what it is useful for or that there is an installable extension. This feature require is aimed just at providing a convenient link for people who already know what Hypothesis is.

### Prior art

Dropbox (see "Install" link):