Restore maintenance.rhtml

from 28c484f1f5e6a009d1b8ff07d85e924d6f947d3c^

This is required for web:disable task - only code for variables and tasks are restored :(

If capistrano will keep the maintenance tasks, it would be better to merge code and templates from  https://github.com/tvdeyen/capistrano-maintenance

(cc @tvdeyen)