Improvements

Updated to use python3 -m instead of pip in Dockerfile
Replaced var with let for variable declarations to enhance code consistency and modernity in a few places
Now automatically loading customization email templates when there is a customization_<customization_key>_templates.html file (helpful for plugins)
Updated customization template loading to work from templates/<plugin_name>/customizations/ too instead of being forced to use the global templates directory (also helpful for plugins)

API

Removed direct dependency on admin app for interlocks to avoid issues when the admin app is not installed
Fixed issue with personal schedule checkbox showing in the calendar when not applicable
Fixed hybrid reservation and use feed not working properly when there are both reservations and use events