New homepage implementation with Activity Feed + Recently Viewed

[agilliland]

This includes a full series of changes to support a new homepage approach ...

• Added a new pub/sub events model in the backend to allow easy publishing and subscribing to different types of events that happen with the app data.
• Created an activity feed which listens for certain published events and records them into a custom table so that we can reconstitute a sense of history of how things happen within a MB instance.
• Added a new view log feature which records each time a user views a Dashboard or Card (currently indicated by api call) so that we can show users their recently viewed stuff.
• Reworked the layout of the Saved Questions tab on the homepage.
• Added the ability to filter the Saved Questions list by database table.

This implements #938 and implements #936

Required:

• icon for happy face in greeting
• icon for Metabase activity
• make sure we are properly assigning colors to multiple users in activity (including 5+ case)
• user and smile column alignment
• border on profile initials (i believe there is a 1px border that we don't want)
• grey line between activity feed initials
• saved questions filter state indicator
• move the ‘ask a question’ button in nav bar and make it white
• fix bug where saved questions listing is not properly sorting by created_at
• color of links in recents sidebar
• color of links in activity subject
• completely remove the ability to save ‘private’ cards and dashboards?
• fix the sorting issue on the activity list
• what to do about broken links to things that don’t exist anymore?
• empty state for recent views list
• empty state for home page activity
• full-height css on sidebar
• clicking an accordion header should collapse the children
• tabs should update the homepage url for a better back button experience and linking
• cursor pointer on table filters
• fix issue where previous table listing is staying visible for a moment when a new database is chosen
• main section of homepage needs full-height as well. if card/activity listing is short things look strange.
• recents need to be aligned left when wrapping to two lines.
• all activity items say "You"
• font size on saved questions (@kdoh)
• empty state centering
• change no results for filter copy to reflect 'no questions have been saved about table yet'
• text for edit on hover

Maybe:

• implement the mosaic background
• tracking a view when a card is created so that the recent views is accurate
• icons on the recent views section (?)
• relative time indicators on recent views section (?)
• truncation of long names on recent views section (?)
• automatically poll for updates to the activity feed

Cleanup:

• remove old angular homepage code

[kdoh]

proptypes

[kdoh]

added some code review comments. i think after that + tests passing we're g2g assuming nothing explodes on proto

[kdoh]

sometimes you press the wrong button cause you haven't had enough coffee

[salsakran]

Can we add hover text for the edit icon on saved questions?

[kdoh]

@salsakran yup. forgot about that one.

[kdoh]

Noticed this is popping up in the console for me locally:
Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of RecentViews. See https://fb.me/react-warning-keys for more information.

[kdoh]