Show images of receipts for each month.
Show table of receipts and balances for each month on finances section.
UI modifications:
-Highlight current section tab on top navigation bar.
-Add icons to home and logout links.
-Add website logo to top navigation bar.
-Add gradient background to top navigation bar.
-Add missing SASS files for various sections.
User.login(...) now expects the apartment parameter to be numeric
UI experience improvement: have the map and a selected payment location always
visible side to side.

Wrap the list of payment locations in a fixed-size scrolling window to
achieve the improvement.