Extract Elasticsearch IndexManager and migrations into its own package

[alexashley]

In #60 we added a mechanism for applying index mappings that includes a method for migrating data after schema changes.

In order to re-use this code in Rode, we need to do a few things:

• move the code into its own Git repository
• change the IndexManager interface and remove the kind-specific methods (e.g., ProjectsIndex)
• update IndexManager.LoadMappings to be agnostic to the document kind
• update the IndexManager.LoadMappings tests, use Go 1.16's MapFS to avoid changing internal state
• expose options/configuration for index prefix, reindex polling interval, and number of poll attempts

[alexashley]

Addressed in https://github.com/rode/es-index-manager/pull/1, https://github.com/rode/grafeas-elasticsearch/pull/80, and https://github.com/rode/rode/pull/77