Cleanup of blob_header component and its underlying components

What does this MR do?

Introduces a cleanup for the app/assets/javascripts/blob/components/blob_header.vue component and other blob-* components to reduce data flow between those to only the required pieces of information.

Main changes:

• Got rid of eventHub and rely on direct events now
• Neither blob_header_viewer_switcher, nor blob_header_default_actions need full-size blob to be passed, so cleaned up the props for both components to reduce data noise in the app
• Since blob_header_viewer_switcher operates with only one prop, converted it to a v-model aware component

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team