Signup 'mandate' mode and other improvements

Summary:
Plans.months
Wallet lock
Skip the payment method selection if there is only one option
Signup: Select the first domain if APP_DOMAIN is not on the list of public domains
Signup: Fallback to 'user' icon for unknown plans
Improve and test User::unrestrict(true)
Signup: Prefer monthly plans on left, yearly on right
Give themes more control over the menu
Allow themes to overwrite UI localization
Small improvement in loading theme localization
Improve menu localization mechanism
Support theme's fonts dir, fix fatal error in localization

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: mollekopf, #hkccp_developers

Differential Revision: https://git.kolab.org/D4136