refactored end_points.js and related code

[rachjiang]

• Split endpoints.js into 3 separate files in “src/back_end/api/“ by categories named leads.js, projects.js, and events.js
• Removed unused firebase methods detected by ESLint in each category’s import
• All files in “Src/back_end/test” have updated import paths
• Updated import paths in “src/api/api.js” to import all query functions from the corresponding categorized endpoint file (i.e. leads.js, projects.js, events.js)
• Replaced all database calls accessing api import due to refactoring of endpoints.js. For example, getActiveLeads() previously accessed api as: api.getActiveLeads(). Now, it’s leads.getActiveLeads();