Stage all changes by default in Web IDE

What does this MR do?

This is a fix for issue #33441 (closed)

Always stage all changes by default. This was already existing for deleting a file. This commit also adds staging by default to the below cases:

• Adding/uploading a new file
• Renaming a file
• Editing a file

Here's a table that outlines the old behaviour and the new behaviour:

Action	Old Result	New Result
Create a new file	File is not staged	File is staged
Upload a file	File is not staged	File is staged
Rename a file	File is not staged	File is staged
Edit a file	File is not staged	File is staged
Delete a file	File is staged	File is staged

This doesn't change the behaviour for trees (folders), since git doesn't allow staging just folders:

Action	Old Result	New Result
Add a new folder	Folder is not staged	Folder is not staged
Rename an empty / newly added folder	Folder is not staged	Folder is not staged

Acceptance Criteria

Testing this branch locally

This change is behind a feature flag :stage_all_by_default. In the rails console (rails c), enter the following command to enable your feature flag:

Feature.enable(:stage_all_by_default)

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

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