Enable Infinite Scroll Up

[mloutraris-ge]

Additionally fix display issue for infinite scroll 'blank block' #2535

Code changes enable infinite scroll up. Changes are as follows:

File: misc/tutorial/212_infinite_scroll.ngdoc
Update documentation to load the data for up and adjust the load data
for down to append to exiting data, not overwrite the hashKeys

src/features/infinite-scroll/js/infinite-scroll.js
Updated file to add new needLoadMoreDataTop method and supporting code
to call method.

src/features/infinite-scroll/test/infiniteScroll.spec.js
Updated unit test to test for needMoreLoadDataTop

src/js/core/constants.js
Added new constant for scroll direction

src/js/core/directives/ui-grid-viewport.js
Added supporting logic to set scroll direction properly

src/js/core/directives/ui-grid.js
Added new function to adjust scroll position after an infinite scroll
data load up or down.

src/js/core/factories/Grid.js
Added logic to reset scroll direction to none after a debounce as well
as increased debounce to 1000 to allow for timeout added to reset scroll
position to execute

src/js/core/factories/GridRenderContainer.js
Added logic too datawatch function to calculate the row to scroll to
after a data load. The up scroll is tricky because you ahve to find the
row you were on in the new data array. This was done by searching for
the hashKey of the row we were on. Supporting code was also added. The
"blank block" issue was fixed here by allowing the logic to execute if
infinite scroll is enabled.

[PaulL1]

This code looks clean and well documented, and has unit tests, thanks for that.

I can see it fiddles with the scrolling a little bit, @swalters you may want to cast an eye over that, it has a specific if clause that checks for infinite scroll being enabled, but I'm aware that we had problems with blank rows in other situations than just infinite scroll - maybe that code should be generalised?

Given the code is localised to infinite scroll - merging.

[PaulL1]

Note that a chunk of this code has been backed out so as to remove the infiniteScroll logic that had crept into core. However, the concepts and intent I think have been retained.