[ADD] custom price options for monthly payment

...ons/0019_donatehomepage_form_monthly_amount_1_and_more.py

.../migrations/0020_donateinfopage_form_amount_1_and_more.py

donate/models.py

donate/static/donate/assets/js/scripts.js

donate/templates/donate/donate_home_page.html

donate/templates/donate/donate_info_page.html

donate/templates/donate/donate_project_page.html

donate/templates/donate/donate_project_page_coalition.html

donate/templates/donate/donate_region_page.html

donate/templates/donate/donate_targeted_donations_page.html

donate/templates/donate/form_amounts_snippet.html

donate/templates/donate/form_monthly_amounts_snippet.html