Add support for adding attachments in Content Editor

What does this MR do?

Adds support for uploading files as attachments in content editor in wikis.

Issue: #335114 (closed)

Screenshots or Screencasts (strongly suggested)

State	Screenshot
Initial state: click on link dropdown
Uploading an attachment (in progress)
Attachment uploaded successfully
Uploading an image (in progress)
Image uploaded successfully
When a link is selected, does not show the upload file option; shows "remove link" instead
Error state: file upload failed

How to setup and validate locally (strongly suggested)

1. Open wikis for any project.
2. Edit or create a new page.
3. Click Use the new editor button to switch to content editor.
4. Click "Upload file" under the link dropdown to upload a file or an image. You can also click "Upload image" under the image dropdown to upload an image.

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.

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 #335114 (closed)