Togglea-able nocache headers for development

[Wiljami]

Right now if your site is a development site, seravo-plugin sends nocache headers. This is feature:

https://seravo.com/docs/configuration/wppalvelu-plugin/#use-nocache-headers-if-the-site-is-in-development-mode

This should be a toggle-able feature within seravo-plugin, so that the development site could be tested with cache.