Replace 'rect' and 'circle' with constants in frontend code

Problem to solve

There are multiple instances of 'rect' and 'circle' in our frontend code. These are used to determine the shape of avatars.

Suggested solution

Use a shared constant instead of the string values.

Implementation

Replace instances of these two strings with a shared constant(s) - could use the ones added in !73334 (comment 719454073).