Add Replace button to Blob Header

What does this MR do?

This is part of a bigger effort to convert the entire BlobHeader from HAML to Vue. In this MR, we implement the replace buttons.

Replace button	Modal

Testing Case

This feature is behind a feature flag, to turn on:

rails c

Feature.enable(: refactor_blob_viewer)

1. Navigate to Repository > Files
2. Click on a file to get to the Blob page
3. Click on the "Replace" button
4. Upload a file

Same Target Branch

Do not change the "Target branch" field. And the file will be replaced on the current branch.

Upload Blob	File Updated

Create a new MR

Enter a different "Target branch" field and select "Start a new merge request with these changes". The uploaded file will make the changes in a new merge request.

Upload Blob	New MR

Follow up MR

Upload blob issues

I am re-using the app/assets/javascripts/repository/components/upload_blob_modal.vue here, but I noticed a few issues with the components.

1. The drag & drop feature doesn't seem to work. But clicking the upload does.
2. There is an Uncaught TypeError once a file is uploaded.

I don't think this should this should dissuade us from re-using this component. It just means it should be improved. But I think we can do this iteratively in another MR. Here's the follow-up to get these fixed > #332734 (closed) 😀

Switch to use GraphQL

The current upload_blob_modal.vue is using REST; ideally we can switch it to use GraphQL. There is an existing issue for the backend to make the port > #330406 (comment 572236568). Once that's done, we can upload this to use GraphQL 🙌

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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

Related to #323210 (closed)