[FIX] Balances widget: Update paddings, fix <dl>

Use correct LESS variables for vertical and horizontal paddings.
Don't render multiple definition lists for each gateway.
[FEATURE] Balances widget: Collapsible currencies

Make currency headers clickable to expand/collapse gateways list.
[FEATURE] Balances widget: Add XRP details

Show Reserve and Available for XRP funds, details collapsed by default.
[FEATURE] Overview page: Split into widgets (RT-2731)

New widget stubs:
- Net Worth
- Asset Allocation
- Markets
- Orders
[TASK] Widgets: Fix headline padding, line, margin
[TASK] Net worth widget: Show selected currency
[FIX] Balances widget: issuer margins, text width
[FEATURE] Balances widget: Animate on show/hide
[TASK] Balances widget: Update design
[FIX] Balances widget: Fix reserve info icon
[TASK] Drop rpAutoMaxHeight directive

Will get replaced by a more generic slider.
[FEATURE] Effects: Add rpSlide directive

Use with div(rp-slide="parameter_to_watch").
[FIX] Balances widget: Fix margins and paddings
[FEATURE] Widgets: Animated show/hide with rpSlide
[FEATURE] Balances widget: Animated currencies show/hide

Uses rpSlide, too.
[FEATURE] Widgets: Animated show/hide caret
[FEATURE] Widgets: Hand cursor for clickable divs

Provides better visual clue that these areas can be clicked to show/hide
parts of the interface.
[FIX] Top-aligned popovers: Arrow border-color
[FIX] Popover directive: Make rpPopoverTitle optional
[FEATURE] Popover directive: Add rpPopoverContent

The value of this HTML attribute is parsed for Angular expressions
in mustaches ($interpolate). Use rp-popover-content if dynamic data
is needed in the popover, keep data-content otherwise (no parsing then).

@vhpoet