Inline browser extension into Bouncer flow

## Feature Request Form
### Problem you are trying to address with this feature

A user is trying to share a direct link to an annotation on a URL which is likely to be behind a paywall or otherwise inaccessible to Via.

### Your solution

Inline the browser extension installation into the user's flow when they click on a Bouncer link to minimize friction. Something like this:

1. User clicks bouncer link
2. Bouncer detects that user does not have extension installed and knows (_via some TBD magic_) that the annotated content is (or probably is) paywalled
3. Bouncer presents a page explaining that the link goes to an annotated page and the user will need to install an extension in order to continue. There could be a big 'Continue' button that displays an inline extension dialog. If the user accepts, the flow continues onto the original URL which activates the extension, otherwise they are given other options.

Original context: https://groups.google.com/a/list.hypothes.is/forum/#!topic/dev/68MiLmmk_0M

No implementation files or tests are named. Start by reviewing the issue and its linked discussion, then define the Bouncer flow, paywall detection, inline extension installation, continuation behavior, and alternative options before identifying the relevant entry points and tests.

Written by the indexing model from the issue text.