Miscellaneous code quality issues

[csande]

• Inline imports when they should be at the top of the file
• Django query optimization (i.e. fetching too much data, missing use of select_related, etc.)
• Unsafe default values or assumptions (e.g. assuming a value for an instance hostname)
• Repeated code which should be helper functions for consistency
• Logical assumptions that may not hold as plugin changes (i.e. unwise use of an if-else statement)
• Incorrectly returning 200 statuses for error responses from SimpleAPI endpoints

Examples highlighted here: https://github.com/canvas-medical/gtm-extensions/pull/97