Add "Copy" button to file view of raw text (#21629)

If a raw text file is displayed, a copy button of the text is enabled.

* Closes #12866

### Before

![image](https://user-images.githubusercontent.com/20454870/198898628-df1bcb0c-79d7-4ffb-95e4-441d77430827.png)

### After

![image](https://user-images.githubusercontent.com/20454870/199988152-ea1099ad-29e1-4765-a9ca-4c03c1737453.png)

#### Rendered files and binaries have their button disabled

![image](https://user-images.githubusercontent.com/20454870/199988408-73de6327-5e9e-462b-b2b6-8c3f5b878386.png)

![image](https://user-images.githubusercontent.com/20454870/199988563-844f8656-f48d-4929-880e-b6558c1c054a.png)

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -88,6 +88,7 @@ edit = Edit
 
 copy = Copy
 copy_url = Copy URL
+copy_content = Copy content
 copy_branch = Copy branch name
 copy_success = Copied!
 copy_error = Copy failed
@@ -1090,6 +1091,7 @@ editor.cannot_edit_non_text_files = Binary files cannot be edited in the web int
 editor.edit_this_file = Edit File
 editor.this_file_locked = File is locked
 editor.must_be_on_a_branch = You must be on a branch to make or propose changes to this file.
+editor.only_copy_raw = You may only copy raw text files.
 editor.fork_before_edit = You must fork this repository to make or propose changes to this file.
 editor.delete_this_file = Delete File
 editor.must_have_write_access = You must have write access to make or propose changes to this file.