Updated several files with the changes mentioned in Bart's code review.

app.js - updated south panel to be adjustable size, adjusted things to account for changes to other classes, switched to using ref instead of id for panels
en.js & es.js - updated i18n with new classes (still needs translating to spanish)
FeatureEditor.js - updated to work with either FeatureEditPanel or FeatureEditPopup, essentially incorporated FeatureEditor_2 changes
FeatureEditorGrid.js - removed reset function as it was almost entirely duplicate code, removed an unneeded check
FeatureEditor_2.js - incorporated into FeatureEditor.js
GeoGitHistory.js - updated some comments, removed some dead code, updated to use the new GeoGitUtil, updated to use i18n
GeoGitRepoInfo.js - updated to use i18n, removed some dead code, switched to using static JSON decode function instead of creating a JSON reader every time
GeoGitUtil.js - moved up out of plugin folder into script folder, changed to be more of a static class, added JSON readers for GeoGit functions so other classes don't have to create new ones, updated parseFeatureType to use split function, store Geogit information in layer metadata instead of actual layer
VersionedEditor.js - changed reset to just create a new FeatureEditorGrid instead of resetting it, made the attr_diff styles configurable, changed the attr_diff layer to only be created once, updated to use new GeoGitUtil, updated i18n, removed buttons from being stored in the class
FeatureEditMixin.js - new mix-in class that is all the duplicate code between FeatureEditPanel and FeatureEditPopup
FeatureEditPanel.js & FeatureEditPopup.js- changed to remove all duplicate code and uses FeatureEditMixin instead