Front-end implementation for editing file links

[JasonChong96]

Problem

Users might want to replace their uploaded files. This PR implements the front-end for replacing files.

Solution

Improvements:

• Fixed margins on url drawer
• Fixed url drawer button border colors
• Extend CollapsibleMessage to be able to be absolute positioned if needed

Before & After Screenshots

AFTER:
Desktop
iPhone X

Tests

1. Replace file with same extension, it should be replaced with the new file
2. Replace file with different extension, long url should change to reflect new extension. Old file should be set to private and new file should be accessible

[yong-jie]

code is lgtm

[yong-jie]

looked through the code, lgtm.