Prior to this fix the text would be incorrect when the current user reacted and number of reactions was above 2.
This commit fixes the bug and also makes the following changes:
- separates text computation in a standalone lib to make it easier to test
- increases the number of displayed usernames in reaction text (from 5 to 15)
- adds a full test suite for this new `getReactionText` function
- fixes a bug in reaction fabricator which would prevent to change the count to zero
