Rename to rollmint

[tzdybal]

• rename GitHub repository
• rename go module
• rename imports
• rename all comments
• rename variables/structs/etc with opti and optimint in their name
• fix links (badges/etc)
• grep entire directory to ensure full rename
• create cosmos-sdk-rollmint fork (also ref: Document the different branches and their workflows celestiaorg/cosmos-sdk#251)
  • remove unnecessary/unrelated branches and tags
• update imports in cosmos-sdk
• rename variables/imports with opti preffix in cosmos-sdk
• release/tag rollmint-enabled cosmos-sdk verrsion
• Update instances of Optimint to Rollmint celestiaorg/docs#249

[tzdybal]

Codewise, everything is done. Docs PR pending.