Add a new faith to use for converting from the Invictus mithra_religion

[IhateTrains]

The converter's current mithraic_mysteries seems to better represent the Roman Mithraism than the Iranian Mithraism.

After discussing with @LinkLuver, we should add a new faith by doing the following:

• use the existing mithraic_mysteries icon, probably some of the loc as well
• copy base game mazdayasna doctrines, tenets and holy sites as a base
• change Xwedodah to Adaptive
• change Righteous to Pluralist
• change Ritual Celebrations to Sun Worship

mithraic_mysteries (in Hellenic religion) can probably be kept inside converter_faiths.txt for people who want to make some custom mappings including this faith, but will be unused by the converter by default for now.