Add support for legacy blob viewers

What does this MR do?

This is part of a bigger effort to refactor the repository browser.

Changes in this MR will enable us to load legacy (HAML) viewers when the :refactor_blob_viewer FF is enabled, giving us the ability to test refactored blob viewers in production with minimal impact.

Screenshots (strongly suggested)

Viewer	Type	Screenshot
Audio	Rich
PDF	Rich
Video	Rich
SVG	Rich
Markdown	Rich
Notebook	Rich
OpenAPI	Rich
Image	Rich
Sketch	Rich
Text	Simple

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #333893 (closed)