A new page has been created for viewing assets. This replaces the old
`selectedToken` state, which previously would augment the home page
to show token-specific information.
The new asset page shows the standard token overview as seen previously
on the home page, plus a history filtered to show just transactions
relevant to that token.
The actions that were available in the old token list menu have been
moved to a "Token Options" menu that mirrors the "Account Options"
menu.
The `selectedTokenAddress` state has been removed, as it is no longer
being used for anything.
`getMetaMetricState` has been renamed to `getBackgroundMetaMetricState`
because its sole purpose is extracting data from the background state
to send metrics from the background. It's not really a selector, but
it was convenient for it to use the same selectors the UI uses to
extract background data, so I left it there for now.
A new Redux store has been added to track state related to browser history.
The most recent "overview" page (i.e. the home page or the asset page) is
currently being tracked, so that actions taken from the asset page can return
the user back to the asset page when the action has finished.
The "i18n-provider" module has been replaced by a new `i18n.js` module
in the `contexts` directory which provides the `t` function via the new
React Context API.
The legacy context API is still used throughout the codebase, so a
legacy context provider has also been added as a shim until we migrate
away from the old API. The migration does require changing every single
place where the `t` function is used, so it is a non-trivial amount of
work. This shim allows us to tackle it one piece at a time without
breaking anything.
This was placed in a new `contexts` directory because it didn't seem
to belong in any existing categories. It certainly isn't a higher-order
component.
