Refgenieserver

Added
- `remotes` section in the refgenieserver config, which supersedes `remote_url_base`. It can be used to define multiple remote data providers.
- new endpoints:
- `/remotes/dict` to get `remotes` dictionary
- `/assets/dir_contents/{genome}/{asset}` to get a JSON object (array of strings) listing all files in the asset directory
- `/assets/file_path/{genome}/{asset}/{seek_key}` to get a *remote* path to the specified file
- direct *unarchived* asset file downloads from asset splash page, both for seek_keys and other asset files

Changed
- endpoints that return digests return plain text instead of JSON objects for easier parsing; [67](https://github.com/refgenie/refgenieserver/issues/67)

Added
- private endpoint serving genomes dict to openAPI schema; [105](https://github.com/refgenie/refgenieserver/issues/105)

Added
- API v3, which is a complete redesign and extension of the previous version
- in `refgenieserver archive`, when a `archive_checksum` is missing for a previously archived asset it will be recalculated.
- response models to API endpoints
- API version tags in the openAPI specification
- a `legacy_archive_digest` to the tag attributes

Changed
- split the user interface into: index, genome and asset pages
- grouped endpoints by API version in the swagger docs
- the archives created with `refgenieserver archive` are now named after digests

Changed
- in `refgenieserver archive`:
- implement safer way of asset archive creation
- follow symlinks

Added
- support for external asset sources via `remote_url_base` key in the config
Changed
- path specified in `genome_archive_config` is considered relative to the refgenie genome config file, unless absolute.
- non-servable assets purging is now performed prior to serving rather than after each archive job completion
- dropped Python 2 support
Removed
- support for old `genome_archive` key; use `genome_archive_folder` and `genome_archive_config` from now on.

Changed
- `refgenieserver archive` requires all assets to be complete prior to archiving