Misc/user panelizer refactor

## Ready State: Yes

Relates to https://imagex.atlassian.net/browse/LDC-261

Refactors imagex_user panelizer layouts and how imagex_user_follow & imagex_activity provide panes to the default layout.

The default user layout has been change to render the basic user data with imagex_user_follow & imagex_activity panes added on.

## Requirements & Dependencies

None

## Deployment Instructions

* Merge code.
* Clear caches with `drush cache-clear all`

## Usage/Test Instructions

* Run installer & ensure a default user panelizer layout is provided (unless override on per project basis)
* Enable imagex_user_follow and ensure the "follow user" block is shown on the default layout
* Enable imagex_activity and ensure the "Activity" block is shown on the default layout