R1:af8cc2a5fc75

Don't select {F...} text after adding it to textareas

Summary:
Fixes T10039. We add text to textareas in two cases:

- Users clicking assitance buttons in Remarkup text areas.
- Drag-and-drop file uploads.

In case (1), it makes sense to highlight the text (it shows the user what we inserted, and lets them undo the action easily if it isn't what they wanted).

In case (2), it does not. Users almost never want to delete or edit a file reference. It is slightly nice to have the reference as a visual…