Add link to install/use Hypothesis client in navbar

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

If users login to their Hypothesis accounts and they have already created at least one annotation, there is no obvious link in the activity pages navigation to get/install Hypothesis in order to annotate a page. For example, this affects users who have:

- Logged in to Hypothesis on a new device which does not have the extension installed
- Have uninstalled the extension but want to install it again
- Be using a browser for which we do not currently provide an extension

### Your solution

Inspiration from other services:

**Pocket**
'+' link shows dialog that allows users to add a new bookmark to their account.


**Dropbox**
Profile menu has an 'Install' link.


Start by locating the activity pages navigation and the existing install or account links. Review how navigation actions and the install destinations are defined, then add an obvious link for users who need to install or use Hypothesis and verify it appears in the relevant activity-page states.

Written by the indexing model from the issue text.