Back / Forward buttons

### What

Ability to navigate back and forward, like a web browser.

### Why

Ever since Simplenote introduced internal links, complex notes could finally be broken up into smaller logical units. Long notes became summaries that link to other notes that expand on specific subjects. These notes, in turn, can be broken up into even more specific notes, and so on.

Internal links is an incredibly useful feature, they promote good structure and organization. Navigating internal links, though, feels rather limited; you can only go forward. The lack of a Back button means that if, for whatever reason, you wanted to go back a note, you have to start from scratch, repeat your search, find the desired note, and retrace all your clicks. This could be as simple as clicking _Back_.

### How

Ideally, navigating notes in Simplenote would work just like navigating pages in a web browser; clicking an internal link would add the current note to the history so that it's possible to go back to it either by clicking _Back_ or pressing Alt+Left. Similarly, clicking _Forward_ or pressing Alt+Right would do the opposite.

Start by tracing how internal links navigate between notes and how keyboard shortcuts are handled. Compare the current note navigation with the requested browser-like history, including Back and Alt+Left and Forward and Alt+Right. Done means linked-note navigation supports both directions without requiring a new search.

Written by the indexing model from the issue text.