[web][native] Rename threadMembers in mentions typeahead code to mentionsCandidates

Summary:
In a later diff I'm going to update the mentions typeahead to sometimes show results that aren't members of the thread, so I want to rename this variable to avoid misleading the reader.

Depends on D6806

Test Plan:
I tested this stack with the following steps on both web and native:

1. Make sure users get auto-added to a sidebar when @-mentioned
2. Test mentions typeahead and make sure it includes all members of parent
3. If a user is @-mentioned in a text message that triggers the creation of a sidebar, then make sure they are included in the sidebar creation message
4. If a user is @-mentioned in a text message that triggers the creation of a sidebar, then make sure their name is bolded from the very start of when the message is visible on-screen

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6807