Implemented global theming

[tbeidlershenk]

Changes

• Created app_theme.dart that defines a global app theme as a ThemeExtension on ThemeData. Details on how to access global theme is explained in this file
• Condensed the BoxDecorations for the containers across the app into the global theme
• Created a color palette for the application with 5 shades
• Condensed the TextStyles across the app into the global theme
• Created theme_manager.dart to allow us to dynamically switch themes. This isn't in use right now but once we implement theme switching (if we do) then it'll be useful
• Created a const_settings.dart file to define the colors for now, probably should switch to JSON for our themes

Notes

• The global theming is meant to do 3~ things: we can change the feel of the whole app at once, to condense the number of styling "choices" we have to make when developing UI components, and to allow for dynamic theme switching for the user.
• Overriding the theming is easy... simply get the theme attribute and call .copyWith(attributeName: attributeValue). Examples are all over the codebase.

[jeffzheng13]

For login_page.dart and signup_page.dart, there seems to be a variable Widget spacing = SizedBox(30). It doesn't seemed to be used so I went ahead and commented it out. Feel free to change it back if needed, but doesn't seem like its being used.