Assign the posts format spinner to an instance variable instead of using findViewById each time we save.

Also added a null and index check when getting post format value. Fixes #1449
Fixes #1451 and #1407 by standardizing the maximum width for bitmaps that we are adding to spans in the editor.

* Made all ImageHelper methods static.
* Changed `getMaximumThumbnailWidth()` in EditPostContentFragment to accommodate for 48dp of padding on each side, instead of where it was being calculated in ImageHelper.getThumbnailForWPImageSpan(). This allows ImageHelper.getScaledBitmapAtLongestSide to also use this to resize to the proper dimensions. This ensures that all bitmaps we are adding to the editor will be the correct size.
* Removed getThumbnailWidth() method since it is no longer needed.