Background events are now sent in the `Background` category, rather
than `backend`. Conventionally we use the term "background" over
"backend", as it's not really a "backend" in the normal sense since
it's a client background process. Also it's capitalized because all of
the other event categories are capitalized as well.
The metrics URL has been updated to use `background` instead of
`backend` as well, for consistency.
Luckily we don't have to worry about our metrics being disjointed due
to this name change, because the background metrics never worked to
begin with! So there will be none under the old name. The metrics will
be made functional in a separate PR.
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.
* Send metrics event from backend for on chain transaction failures
* Passes state object to backEndMetaMetricsEvent, and adds getMetaMetricState selector
