Fix margin of message boxes below page header
Rename .admin-page-* to .content-page-*

Also moved it from admin.scss to main.scss
It's not only useful for admin pages but also e.g. in some event
management areas.
Include admin.scss on all admin pages
Add payment blueprint, menu item and basic WP/RH
Add settings (only currency so far)
Add event-specific settings (EventSettingProxy)
Change url for legacy epayment settings
Add event-specific payment settings
Use the email messages from the new settings

Also removed a useless check that was always true
Use the new payment enabled setting
Create and apply i-data-list CSS class
Use the switch widget to enable epayment
Fix switch inside i-data-list
Add default currency setting, copy defaults

The defaults are only copied when the payment module is enabled for the
first time for an event
Define __all__ in indico.modules.payment
Mark currencies as required
Add missing default setting values
Add IndicoPlugin.event_settings
Disable HTML5 validation for UsedIf fields

Chances are good that those fields are required, but only if e.g. a
certain checkbox is checked.
Add basic payment plugin settings