Improve code quality

[mtnorthcott]

Reserved for Catalyst Academy 2021

We would like to analyse and find potential improvements in code quality throughout the GeoNode codebase. The following improvements have been proposed and should be addressed by this epic:

• Replace body of Layer.display_type with a conditional
• Move constants to top of file
• Fetch dictionary keys with dict.get(...) where appropriate to provide default value
• Remove redundant method overrides
• Prefer sets for in-line lookups
• Refactor long conditional statements
• Prefer generators as input to compatible functions
• Remove redundant enumerate calls
• Remove duplicate function calls
• Remove or replace dependencies
• Replace instances of print with equivalent logger statements
• Add docstrings to functions where it is missing
• Investigate @cached_property and where it could be used
• Remove python_resize_image library if we can do ops in PIL
• Simplify conditional logic
• Replace magic numbers and calculations where possible
• Add helper function to perform encode-decode operations
• Catch specific exceptions
• Remove Python 2/3 cross-compatible code
• Implement GET and POST handler functions
• Close open file descriptors
• Replace existing string formatting with f-strings